Fix path resolution for child mods of those expanded by `include!`
Child modules wouldn't use the correct candidate paths due to a branch that doesn't seem to be doing what it's intended to do. Removing the branch fixes the problem and all existing test cases pass.
Having no knowledge of how any of this works, I believe this fixes#17645. Using another test that writes the included mod directly into `lib.rs` instead, I found the difference can be traced to the candidate files we use to look up mods. A separate branch for if the file comes from an `include!` macro doesn't take into account the original mod we're contained within:
```rust
None if file_id.macro_file().map_or(false, |it| it.is_include_macro(db.upcast())) => {
candidate_files.push(format!("{}.rs", name.display(db.upcast())));
candidate_files.push(format!("{}/mod.rs", name.display(db.upcast())));
}
```
I'm not sure why this branch exists. Tracing the branch back takes us to 3bb9efb but it doesn't say *why* the branch was added. The test case that was added in this commit passes with the branch removed, so I think it's just superfluous at this point.
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