Use common routines for pip install and pip sync

[charliermarsh]

Summary

This PR takes the functions used in pip install, moves them into a common module, and then replaces all the pip sync logic with calls into those functions. The net effect is that pip install and pip sync share far more code and demonstrate much more consistent behavior.

Closes #3555.

[charliermarsh]

The test changes are all in the logging...

Previously, we logged uninstalls in pip sync but not pip install; now we log in both.

Previously, we logged resolution in pip install but not pip sync; now we log in both.

I don't feel that strongly about either.

[charliermarsh]

Ugh, this is surfacing a really subtle bug in a specific case: we have an editable installed, and a user requests a package of the same name, and so we resolve to the already-installed editable during resolution, but it then gets marked as extraneous when we create the install plan.