Rollup merge of #158372 - mkroening:rustfmt-cfg_select-modules, r=ytmimi,jieyouxu

rustfmt: Discover modules via `cfg_select!`

This PR renames all occurrences of `cfg_match!` in rustfmt to `cfg_select!`. This makes the module file detection logic from rust-lang/rustfmt#6522 kick in for `cfg_select!` instead of `cfg_match!`, which no longer exists.

This PR performs no other adjustments to the logic to be as small as possible.

I am opening this PR in this repo since that is also the target for the more comprehensive rust-lang/rust#154202, which covers more than merely detecting other files through `cfg_select!`.

Closes rust-lang/rust#158371.

CC: @ytmimi, @CAD97

Author: jhpratt