Better handling of local file install edge cases; handle local file extras

[matteius]

We definitely don't want relative one directory to be treated as package name and pip documentation does not provide much guidance on this.

The issue

Fixes #5917

The fix

If the path is not absolute, ensure it is prepended with ./ so it is treated as an installable path by pip (and not a named requirement) -- also handle the specification of extras with local file installs.

The checklist

• Associated issue
• 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 #.

[matteius]

Well it seems this one requires more work to untangle the edge cases.

[oz123]

Reading the original issue, I see a Pipfile from the user. But the dependencies sections are empty.
Can we add a test case for this PR which uses a Pipfile that walks the code path added here?
For the sake of future maintainers and us, I believe it's worth the effort.

[matteius]

@oz123 its a github actions reproduction repo, I want to extrapolate the test package into our test runner to help check editable installs better: https://github.com/stewartmiles/pipenv_editable_install_repro/blob/main/try_pipenv_versions.sh

[matteius]

I discussed with @oz123 and he is not going to block me from merging this, given both of our time constraints, I am going to make a concerted effort to improve pipenv test suite during hacktoberfest (October time-frame)