Fix detection of symlinked Cargo.tomls

This commit fixes a bug introduced in #276 where symlinked Cargo.tomls
wouldn't get detected due to readDir() reporting them as - well -
_symlinks_ instead of regular files.

Note that we continue to keep the `if type == "directory"` condition for
recursion which means that we'll fail to detect "nested" Cargo.tomls if
someone decides to go wild and create workspace with symlinked crates.

That I'm not 100% sure on how to approach, but fortunately cases like
those seem to be practically non-existent (and Crane apparently not
supporting them builds a bit of confidence here as well).

Closes #280.

config.nix

@@ -306,7 +306,7 @@ let
                 path = "${root}/${dir}/${name}";
 
               in
-              if type == "regular" && name == "Cargo.toml" then
+              if name == "Cargo.toml" then
                 [{ name = dir; toml = readTOML path; }]
               else if type == "directory" then
                 findCargoTomls "${dir}/${name}"

test/fast/default.nix

@@ -13,6 +13,7 @@ args: {
   readme = import ./readme args;
   simple-dep = import ./simple-dep args;
   simple-dep-patched = import ./simple-dep-patched args;
+  symlinks = import ./symlinks args;
   workspace = import ./workspace args;
   workspace-build-rs = import ./workspace-build-rs args;
   workspace-patched = import ./workspace-patched args;

test/fast/symlinks/default.nix

@@ -0,0 +1,8 @@
+{ naersk, pkgs, ... }: {
+  default = naersk.buildPackage {
+    src = pkgs.symlinkJoin {
+      name = "src";
+      paths = [ ./fixtures ];
+    };
+  };
+}

test/fast/symlinks/fixtures/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "app"
+version = "0.1.0"

test/fast/symlinks/fixtures/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "app"
+version = "0.1.0"
+edition = "2018"

test/fast/symlinks/fixtures/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}