Support bare local file paths in dependency parsing

[Mirochill]

Pull Request Check List

Resolves: #6951

Summary

• accept an existing local file like demo.whl as a path dependency even when the argument has no directory separator
• keep separator-less directory names on the simple-package path, so this only broadens the file case reported in the issue
• add a regression test for a bare local wheel filename

Validation

• Not run locally

• Added tests for changed code.

• Updated documentation for changed code.

[sourcery-ai]

Hey - I've left some high level feedback:

• The new parse() logic now hits the filesystem for every separator-less requirement; consider limiting the early _parse_path check to names that look like files (e.g., known archive extensions) to avoid unnecessary stat calls for typical package specs.
• In _parse_path, is_relative_file is a subset of is_relative_path, so the condition (is_explicit_path && is_relative_path) || is_relative_file can be simplified (or commented) to make the intended cases clearer and reduce mental overhead when reasoning about path handling.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `parse()` logic now hits the filesystem for every separator-less requirement; consider limiting the early `_parse_path` check to names that look like files (e.g., known archive extensions) to avoid unnecessary `stat` calls for typical package specs.
- In `_parse_path`, `is_relative_file` is a subset of `is_relative_path`, so the condition `(is_explicit_path && is_relative_path) || is_relative_file` can be simplified (or commented) to make the intended cases clearer and reduce mental overhead when reasoning about path handling.

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[radoering]

I think the sourcery feedback in #10911 (review) makes sense and should be addressed.

[Mirochill]

Thanks, addressed in the latest branch updates. The parser now only probes local paths before or during fallback parsing when the requirement is explicit path-like or a known package archive filename, so plain package names avoid the filesystem path check. _parse_path() also uses the simplified relative/absolute path condition. The latest GitHub Actions matrix, Cirrus FreeBSD, and pre-commit.ci checks are green on 887f055.