Parse Python Core Metadata files (METADATA and PKG-INFO)

[andrew]

I'm inspecting Python distribution archives without installing packages or executing setup.py. Their *.dist-info/METADATA and PKG-INFO files contain identity, dependency declarations, and package descriptions, but Parse does not recognise either format.

At library revision 04d388904f3439cbabf24ac986353a7f12266086, using Go 1.27.1:

data := []byte("Metadata-Version: 2.1\nName: example\nVersion: 1.0.0\nSummary: Currency conversion\nRequires-Dist: requests>=2\n\nPackage documentation.\n")
for _, name := range []string{"example-1.0.0.dist-info/METADATA", "example-1.0.0/PKG-INFO"} {
    result, err := manifests.Parse(name, data)
    fmt.Printf("file=%s result=%+v error=%v\n", name, result, err)
}

Both calls return UnknownFileError. This is a library call; no git-pkgs CLI is involved. The same result occurs with real wheel metadata, including Jupyter and python-telegram-bot.

Please recognise Python Core Metadata and return package identity and Requires-Dist declarations. Descriptive output should preserve Summary, Keywords, Home-page, repeated Project-URL headers, Description-Content-Type, and the description body. Cover folded headers, Unicode, optional fields, and older descriptions stored in a Description header. Parsing should remain a pure function of the supplied bytes.

Add public Parse tests for wheel and sdist member paths, including dependency markers and extras. Descriptive output is tracked at #101; broader distribution-format work is tracked at #80.