Virtual std module subdirectories (#14040)

# Description

Uses "normal" module `std/<submodule>/mod.nu` instead of renaming the
files (as requested in #13842).

# User-Facing Changes

No user-facing changes other than in `view files` results. Imports
remain the same after this PR.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

Also manually confirmed that it does not interfere with nupm, since we
did have a conflict at one point (and it's not possible to test here).

# Performance Tests

## Linux

### Nushell Startup - No config

```nu
bench --pretty -n 200  { <path_to>/nu -c "exit" }
```

| Release | Startup Time |
| --- | --- |
| 0.98.0 | 22ms 730µs 768ns +/- 1ms 515µs 942ns
| This commit | 9ms 312µs 68ns +/- 709µs 378ns
| Yesterday's nightly | 9ms 230µs 953ns +/- 9ms 67µs 689ns

### Nushell Startup - Load full standard library

Measures relative impact of a full `use std *`, which isn't recommended,
but worth tracking.

```nu
bench --pretty -n 200  { <path_to>/nu -c "use std *; exit" }
```

| Release | Startup Time |
| --- | --- |
| 0.98.0 | 23ms 10µs 636ns +/- 1ms 277µs 854ns
| This commit | 26ms 922µs 769ns +/- 562µs 538ns
| Yesterday's nightly | 28ms 133µs 95ns +/- 761µs 943ns
| `deprecated_dirs` removal PR * | 23ms 610µs 333ns +/- 369µs 436ns

\* Current increase is partially due to double-loading `dirs` with
removal warning in older version.

# After Submitting

Still TODO - Update standard library doc

crates/nu-std/src/lib.rs

@@ -5,55 +5,85 @@ use nu_parser::parse;
 use nu_protocol::{
     debugger::WithoutDebug,
     engine::{FileStack, Stack, StateWorkingSet, VirtualPath},
-    report_parse_error, PipelineData,
+    report_parse_error, PipelineData, VirtualPathId,
 };
 use std::path::PathBuf;
 
+fn create_virt_file(working_set: &mut StateWorkingSet, name: &str, content: &str) -> VirtualPathId {
+    let sanitized_name = PathBuf::from(name).to_string_lossy().to_string();
+    let file_id = working_set.add_file(sanitized_name.clone(), content.as_bytes());
+
+    working_set.add_virtual_path(sanitized_name, VirtualPath::File(file_id))
+}
+
 pub fn load_standard_library(
     engine_state: &mut nu_protocol::engine::EngineState,
 ) -> Result<(), miette::ErrReport> {
     trace!("load_standard_library");
+
+    let mut working_set = StateWorkingSet::new(engine_state);
+    // Contents of the std virtual directory
+    let mut std_virt_paths = vec![];
+
+    // std/mod.nu
+    let std_mod_virt_file_id = create_virt_file(
+        &mut working_set,
+        "std/mod.nu",
+        include_str!("../std/mod.nu"),
+    );
+    std_virt_paths.push(std_mod_virt_file_id);
+
+    // Submodules/subdirectories ... std/<module>/mod.nu
+    let mut std_submodules = vec![
+        // Loaded at startup - Not technically part of std
+        ("mod.nu", "std/core", include_str!("../std/core/mod.nu")),
+        // std submodules
+        ("mod.nu", "std/assert", include_str!("../std/assert/mod.nu")),
+        ("mod.nu", "std/bench", include_str!("../std/bench/mod.nu")),
+        ("mod.nu", "std/dirs", include_str!("../std/dirs/mod.nu")),
+        ("mod.nu", "std/dt", include_str!("../std/dt/mod.nu")),
+        (
+            "mod.nu",
+            "std/formats",
+            include_str!("../std/formats/mod.nu"),
+        ),
+        ("mod.nu", "std/help", include_str!("../std/help/mod.nu")),
+        ("mod.nu", "std/input", include_str!("../std/input/mod.nu")),
+        ("mod.nu", "std/iter", include_str!("../std/iter/mod.nu")),
+        ("mod.nu", "std/log", include_str!("../std/log/mod.nu")),
+        ("mod.nu", "std/math", include_str!("../std/math/mod.nu")),
+        ("mod.nu", "std/util", include_str!("../std/util/mod.nu")),
+        ("mod.nu", "std/xml", include_str!("../std/xml/mod.nu")),
+        // Remove in following release
+        (
+            "mod.nu",
+            "std/deprecated_dirs",
+            include_str!("../std/deprecated_dirs/mod.nu"),
+        ),
+    ];
+
+    for (filename, std_subdir_name, content) in std_submodules.drain(..) {
+        let mod_dir = PathBuf::from(std_subdir_name);
+        let name = mod_dir.join(filename);
+        let virt_file_id = create_virt_file(&mut working_set, &name.to_string_lossy(), content);
+
+        // Place file in virtual subdir
+        let mod_dir_filelist = vec![virt_file_id];
+
+        let virt_dir_id = working_set.add_virtual_path(
+            mod_dir.to_string_lossy().to_string(),
+            VirtualPath::Dir(mod_dir_filelist),
+        );
+        // Add the subdir to the list of paths in std
+        std_virt_paths.push(virt_dir_id);
+    }
+
+    // Create std virtual dir with all subdirs and files
+    let std_dir = PathBuf::from("std").to_string_lossy().to_string();
+    let _ = working_set.add_virtual_path(std_dir, VirtualPath::Dir(std_virt_paths));
+
+    // Load prelude
     let (block, delta) = {
-        let std_dir = PathBuf::from("std");
-
-        let mut std_files = vec![
-            // Loaded at startup
-            ("core", include_str!("../std/core.nu")),
-            // std module - Loads all commands and submodules
-            ("mod.nu", include_str!("../std/mod.nu")),
-            // std submodules
-            ("assert", include_str!("../std/assert.nu")),
-            ("bench", include_str!("../std/bench.nu")),
-            ("dirs", include_str!("../std/dirs.nu")),
-            ("dt", include_str!("../std/dt.nu")),
-            ("formats", include_str!("../std/formats.nu")),
-            ("help", include_str!("../std/help.nu")),
-            ("input", include_str!("../std/input.nu")),
-            ("iter", include_str!("../std/iter.nu")),
-            ("log", include_str!("../std/log.nu")),
-            ("math", include_str!("../std/math.nu")),
-            ("util", include_str!("../std/util.nu")),
-            ("xml", include_str!("../std/xml.nu")),
-            // Remove in following release
-            ("deprecated_dirs", include_str!("../std/deprecated_dirs.nu")),
-        ];
-
-        let mut working_set = StateWorkingSet::new(engine_state);
-        let mut std_virt_paths = vec![];
-
-        for (name, content) in std_files.drain(..) {
-            let name = std_dir.join(name);
-
-            let file_id =
-                working_set.add_file(name.to_string_lossy().to_string(), content.as_bytes());
-            let virtual_file_id = working_set.add_virtual_path(
-                name.to_string_lossy().to_string(),
-                VirtualPath::File(file_id),
-            );
-            std_virt_paths.push(virtual_file_id);
-        }
-
-        let std_dir = std_dir.to_string_lossy().to_string();
         let source = r#"
 # Prelude
 use std/core *
@@ -67,8 +97,6 @@ use std/deprecated_dirs [
 ]
 "#;
 
-        let _ = working_set.add_virtual_path(std_dir, VirtualPath::Dir(std_virt_paths));
-
         // Add a placeholder file to the stack of files being evaluated.
         // The name of this file doesn't matter; it's only there to set the current working directory to NU_STDLIB_VIRTUAL_DIR.
         let placeholder = PathBuf::from("load std/core");

crates/nu-std/std/core/mod.nu

@@ -1,4 +1,4 @@
-use dt [datetime-diff, pretty-print-duration]
+use std/dt [datetime-diff, pretty-print-duration]
 
 # Print a banner for nushell with information about the project
 export def banner [] {