Rollup merge of #143550 - petrochenkov:lessmutres, r=lcnr

resolve: Use interior mutability for extern module map

Module map for extern modules is a lazily populated cache, it's not *significantly* mutable.
If some logic in name resolver is parallelized, then this cache can be populated from any thread, and without affecting results of any speculative resolution.

Unblocks rust-lang/rust#143884.
This is a part of [#gsoc > Project: Parallel Macro Expansion](https://rust-lang.zulipchat.com/#narrow/channel/421156-gsoc/topic/Project.3A.20Parallel.20Macro.20Expansion/with/527348747).
cc `@LorrensP-2158466`

Author: matthiaskrgr