Suggest a way of cleaning dist if non-empty

[webknjaz]

This is extracted from #7654.

[chrahunt]

Rebased on master to fix CI.

[pradyunsg]

I have no issues with this merging if someone else in @pypa/pip-helpers or @pypa/pip-committers think this is useful, but I don't think I'd be merging this in myself.

As worded, the new suggestion seems to nudge toward deleting dist/ without taking a look. It's easy enough to automate the deleting of an unnecessary directory but I'd figured that we should not be deleting dist/ by default here. That's because I prefer automation that avoids destructive actions in situations where it's not known what the cause for the fault is or what the correct corrective action should be (do they care about what's in dist/? should they? how can we know? can the action be undone?).

OTOH, I'm sure that someone making a pip release would know what to do to delete a folder, so providing a suggestion isn't required. Plus, some part of me looks at the git command here and it is not clear what it does or why this is the correct suggestion. FWIW, it is the only cross platform suggestion that I could think of too.

[webknjaz]

@pradyunsg that's why I added -i there to make it interactive. So Git will basically ask to confirm deleting things.

[webknjaz]

Also that's why it says "you can", not "you must"

[pradyunsg]

Ah well. The interactive bit does sound useful -- I just tried it locally and, yea, makes sense. Me likey this now.