Auto merge of rust-lang#74275 - wesleywiser:break_up_partitioning_rs, r=pnkfelix

Refactor the partitioning module to make it easier to introduce new algorithms

I've split the `librustc_mir::monomorphize::partitioning` module into a few files and introduced a `Partitioner` trait which allows us to decouple the partitioning algorithm from the code which integrates it into the query system. This should allow us to introduce new partitioning algorithms much more easily. I've also gone ahead and added a `-Z` flag to control which algorithm is used (currently there is only the `default`).

I left a few comments in places where things might be improved further.

r? @pnkfelix cc @rust-lang/wg-incr-comp

Author: bors