break out the upvar code into modules

[nikomatsakis]

I think this code would be cleaner if we made an upvar module with submodules like:

• rustc_typeck::check::upvar::collect_uses
• rustc_typeck::check::upvar::minimization

and just had the "top-level logic" in rustc_typeck::check::upvar.

I also think we should include a comment with the "abstract algorithm" -- but maybe it should live in the reference, actually, and we should just reference to it from here.

Originally posted by @nikomatsakis in rust-lang/rust#86995 (comment)