Some improvemets in requirementslib for clarity

[vshulcz]

The issue

This PR addresses a couple of areas in the code where improvements in argument handling and path management were needed. This PR does not fix a specific GitHub issue but addresses overall code quality.

The fix

Removed an extra space after -i, added some mini docstrings, improved the path handling logic.

The checklist

• [x] Associated issue: No specific issue, general improvement.
• [x] A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.

[oz123]

@vshulcz can you please add a news file?

[matteius]

I'd be fine merging this as-is considering the scope of the other changes and the comprehensive news fragment I added there. Just to be able to cut a release sooner 😅