Auto merge of #120497 - compiler-errors:modulize, r=lcnr

Move predicate, region, and const stuff into their own modules in middle

This PR mostly moves things around, and in a few cases adds some `ty::` to the beginning of names to avoid one-off imports.

I don't mean this to be the most *thorough* move/refactor. I just generally wanted to begin to split up `ty/mod.rs` and `ty/sty.rs` which are huge and hard to distinguish, and have a lot of non-ty stuff in them.

r? lcnr

Author: bors