feat: support local package artifacts in requirements

[anish-sahoo]

Summary

• support simple local wheel/source archive paths in build.python_requirements
• stage referenced artifacts through the existing cog_build context before pip install
• document supported requirements syntax and add unit/integration coverage

Fixes #1945

Supported examples

Users can list local artifacts directly in their requirements file:

./dist/mylib-0.1.0-py3-none-any.whl
./vendor/helperlib.zip
./packages/localpkg.tar.gz

Paths are resolved relative to the requirements file and must point to regular files inside the project. Cog stages those files before the generated pip install -r /tmp/requirements.txt layer, without copying the full source tree earlier.

Not supported in this PR:

• build.run: pip install ./artifact.zip
• local package directories
• name @ file:...
• local --find-links directories
• recursive local requirements includes
• inline hashes/options on local artifact lines

[ask-bonk]

LGTM

github run