Pipenv install refactor (draft)

[matteius]

Thank you for contributing to Pipenv!

The issue

It has been long requested that: pipenv install have the same behavior as pipenv sync when no packages are requested and pipenv install package has the behavior of pipenv update which means only pipenv lock will regenerate the whole lock.

The fix

This is the start of that change.

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 #.