3905 pip version check cache dir

[tomfbiz]

Fixes #3905
Changed rules on where pip version lock file selfcheck.json is stored.

Previously, the lock file was either stored at USER_CACHE/selfcheck.json,
or for vitual enviorments it is stored at sys.prefix\pip-selfcheck.json

Now regardless of whether the user is in a virtual environment, the lock file
is stored either in selfcheck.json in the path specified in cache-dir in
pip.conf, or if none is specified it is stored atUSER_CACHE/selfcheck.json

[dstufft]

Two fairly minor comments, but once those are addressed this looks good to me.

[dstufft]

I'd probably rename this to just SelfCheckState or similar, since it's no longer "global" vs "virtualenv".

[dstufft]

While I applaud you for writing such an extensive news file entry, I think that it is probably overly descriptive compared to the level of impact the typical user is going to see. For most people this will be entirely transparent. I would suggest trimming this down to 1, maybe two sentences. For a look at our existing change log entries, check out https://github.com/pypa/pip/blob/master/NEWS.rst.

[pradyunsg]

There's also a note about this in https://github.com/pypa/pip/blob/master/docs/development.rst#adding-a-news-entry

In order to maintain a consistent style in the NEWS.rst file, it is preferred to keep the news entry to the point, in sentence case, shorter than 80 characters and in an imperative tone -- an entry should complete the sentence "This change will ...". In rare cases, where one line is not enough, use a summary line in an imperative tone followed by a blank line separating it from a description of the feature/change in one or more paragraphs, each wrapped at 80 characters. Remember that a news entry is meant for end users and should only contain details relevant to an end user.

[dstufft]

Looks like you have some test failures, but the code itself looks good to me. Happy to merge this once CI is green.

[lock]

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