Add a --remove-untracked option to the install command. (#2172)

* Add a --keep-untracked option to the install command.

* Call the option --remove-untracked instead.

* Add test and fix Installer.

* Add documentation.

* Make sure to never remove essential packages like pip, setuptools and the root.

* Move logic to the Solver instead.

* Add a few unit tests for Solver.

* Add type hints to solver.py.

Co-authored-by: Steph Samson <hello@stephsamson.com>

* Run black after commit fron Github.

* Import the identifiers used in type annotations.

Co-authored-by: Steph Samson <hello@stephsamson.com>