Have meta.get_field('pk') return the primary key field directly

[Josh Schneier]

Follow up ticket discussion on this topic: ​https://github.com/django/django/pull/8148

While fixing #27897, which is caused by calling .get_field('pk'), I ran across the fact that we are now checking == 'pk' before calling .get_field in several places. My proposal is to simply handle this special case inside of get_field directly.

I have a patch but there is a single test regression on ​this system check. My idea was to change the message and obj to self.model but I'm not sure what the backwards compatibility guarantees are for the message.

Regardless I think we should do this because it will prevent a whole class of bugs in the future and 'pk' is already special cased everywhere.

[Josh Schneier]

Patch here: ​https://github.com/django/django/pull/8191

[Tim Graham]

The discussion on the PR suggests that a DevelopersMailingList discussion is needed to see if there's consensus to proceed with this.

[Simon Charette]

I think this can be closed as wontfix given the discussion on the mailing list didn't reach consensus around the backward compatibility risks.