Rollup merge of #72586 - lcnr:winner-winnowing, r=nikomatsakis

split select into submodules

https://github.com/rust-lang/rust/blob/a0f06d11ae7fe29c8e263e2a5d8cd41b372f0283/src/librustc_trait_selection/traits/select.rs#L1

I extracted two submodules:

- confirmation: apart from `pub(super) fn confirm_candidate`, everything else is private
- candidate_assembly: exports `pub(super) fn candidate_from_obligation` and `pub(super) fn assemble_candidates`

I tried to change as little as possible while doing this and hopefully split this into well reviewable commits.