Rustfmt cannot find mod with #[path] on windows

Rustfmt on windows cannot resolve module to file if #[path] used.

Note, #[path] should contain windows-unfriendly relative path.

To Reproduce

Use #[path = "./some-relative-path.rs"] in code.
Pointed file should be and valid rust source or empty.

#[path = "../module_foo.rs"]
mod module_foo;

Error:

> cargo fmt -- --check
error: couldn't read \\?\D:\a\rustfmt-issue-4477\rustfmt-issue-4477\src\..\module_foo.rs: The filename, directory name, or volume label syntax is incorrect. (os error 123)
Error writing files: failed to resolve mod `module_foo`: \\?\D:\a\rustfmt-issue-4477\rustfmt-issue-4477\src\..\module_foo.rs does not exist

Check out demo for this issue. Also there is two cases reproduced on GHA:

1. this error "file not found"
2. all is ok, #[path] with just filename

Expected behavior

No rustfmt's IO errors.
All modules resolved.

Meta

• rustfmt versions:
  • rustfmt 1.4.15-stable (530eadf4 2020-06-02)
  • rustfmt 1.4.15-nightly (aedff61f 2020-05-19)
• From where did you install rustfmt?: rustup, crates.io
• How do you run rustfmt: rustfmt, cargo-fmt

Start by reproducing the Windows failure with the issue's #[path = "../module_foo.rs"] example and cargo fmt -- --check, using the linked demo repository. Trace rustfmt's module-resolution entry point for Windows paths; done means the relative-path module resolves without an I/O error, while the filename-only case continues to work.