Eliminate base_command.py's dependence on PipSession

[cjerdonek]

This is a follow-on to PR #6835 that finishes removing base_command.py's dependency on index.py / PackageFinder, PipSession, and pip_version_check, etc.

This is again related to #4768 ("Speedup startup time").

[pradyunsg]

It'd be handy to add tests to verify that handle_pip_version_check is called in the correct scenarios.

[cjerdonek]

It'd be handy to add tests to verify that handle_pip_version_check is called in the correct scenarios.

@pradyunsg Okay, I added some tests like this as you requested. I added--

1. A test of the base Command class in test_base_command.py, checking that handle_pip_version_check() is indeed called by command.main(), and

2. Tests of each of the IndexGroupCommand commands (download, install, list, and wheel), testing whether handle_pip_version_check() calls pip_version_check(), for each of the possible combinations of values of options.disable_pip_version_check and options.no_index.

[pradyunsg]

Hah! You're awesome @cjerdonek!