Fixes #1509 short-circuit of installing packages when already installed

[ayemiller]

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Summary of changes

Updates logic of install.py to continue installing packages if one is already installed.

Test plan

Added tests to exercise the bug and demonstrate fix:
test_install_three_packages_with_first_already_installed
test_install_two_packages_with_first_two_already_installed

Tested by running

nox -s tests-3.12 -- -k test_install

[chrysle]

Thanks for catching! Probably, we don't need two tests, though. Besides, the test function names don't really correspond to the actual test procedure..

[ayemiller]

Thanks for catching! Probably, we don't need two tests, though. Besides, the test function names don't really correspond to the actual test procedure..

Whoops! Updated :)