drop require lock when not needed during loading to allow parallel precompile loading (JuliaLang#56291)

Fixes `_require_search_from_serialized` to first acquire all
start_loading locks (using a deadlock-free batch-locking algorithm)
before doing stalechecks and the rest, so that all the global
computations happen behind the require_lock, then the rest can happen
behind module-specific locks, then (as before) extensions can be loaded
in parallel eventually after `require` returns.

Author: vtjnash