Revert defaulting to in-place builds for local directories

[pradyunsg]

Toward #7555
Reopens (😞) #7549, #6276, #3500, #2195

[pradyunsg]

TODO: rebase on master, for fixing CI.

[sbidoul]

Technically, the revert seems ok. Thanks @pradyunsg!

Still wondering if a good communication campaign about the change would not be better than a revert. Difficult choice.

[pradyunsg]

I thought most of us were in agreement about this, that our best course of action is revert and re-evaluate. Plus there's been a "big blob of text" (#8165 (comment)) communicating to users that -- "we'll be reverting the "build in place" behavior change in a pip 20.1.1 release".

I think the ship has sailed on "if" we revert this now -- we will -- but I don't think there's any reason we can't reintroduce this in pip 20.2 with a better (read: more thought out, less rushed because of @pradyunsg) rollout plan. :)

[pfmoore]

I'm not entirely sure how best to review something like this (I assume you used git revert, and I certainly haven't checked line for line that it correctly reverted the commits it should have done) but in general this LGTM.