pip install breaks existing install (and prints that it did so) instead of refusing to perform

[1ace]

Environment

• pip version: 20.3.3
• Python version: 3.9.1
• OS: python docker image

Description

When running pip install with packages that are incompatible, pip breaks the existing install instead of refusing the action.

Breaking the install should either be forbidden, or hidden behind --force if you think someone out there really wants that behaviour, but it should definitely not be the default behaviour...

Expected behavior

pip should refuse to break the existing install, instead of casually mentioning it just did and exit with 0.

How to Reproduce

$ docker run --rm -it python bash

# pip install scanpydoc==0.5.5
[...]
# echo $?
0
# pip list | grep Sphinx
Sphinx                        3.4.3

# pip install sphinx==1.8.4
Installing collected packages: sphinx
  Attempting uninstall: sphinx
    Found existing installation: Sphinx 3.4.3
    Uninstalling Sphinx-3.4.3:
      Successfully uninstalled Sphinx-3.4.3
ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
scanpydoc 0.5.5 requires sphinx>=3.0, but you have sphinx 1.8.4 which is incompatible.
Successfully installed sphinx-1.8.4
# echo $?
0
# pip list | grep Sphinx
Sphinx                        1.8.4
# pip list | grep scanpydoc
scanpydoc                     0.5.5

[uranusjr]

See #7744.