Add 'pip cache dir' command to show cache directory

[hugovk]

Fixes #7350.

Print out the path to pip's cache directory, which is different depending on OS.

For example on macOS:

$ pip cache dir
/Users/hugo/Library/Caches/pip
$ pip cache dir 123
ERROR: Too many arguments

This will allow CIs like GitHub Actions to cache pip downloads with much less config, hopefully increasing use of caching, and should reduce the load and cost of running PyPI.

Others would benefit from this, and some have been resorting to using pip's private, internal API:

• https://stackoverflow.com/a/48956368/724176
• https://pip.pypa.io/en/latest/user_guide/#using-pip-from-your-program

[duckinator]

I think a lot of folks will benefit from having a way to easily determine the cache directory, especially in CI environments. 🙂

[pradyunsg]

A test hitting this branch would be great! :)

[hugovk]

👍 Test added in 6b640d1

[pradyunsg]

LGTM, other than a non-blocking comment for an additional test.

[pradyunsg]

puts on release manager hat

This looks like a useful change, enhancing a feature we've got in the beta. OK to put this into the 20.1 release.