add logic to prefer loading modules that are already loaded

[vtjnash]

Iterate over the list of existing loaded modules for PkgId whenever loading a new module for PkgId, so that we will use that existing build_id content if it otherwise passes the other stale_checks.

[vtjnash]

Tested locally with

JULIA_DEPOT_PATH=testdepot ./julia -e 'using REPL'

then

JULIA_DEPOT_PATH=testdepot: ./julia -e 'Base.require_stdlib(Base.PkgId(Base.UUID("3fa0cd96-eef1-5676-8a61-b3b8758bbffb"), "REPL")); using REPL; display(Base.loaded_precompiles)'

which shows the result is correct for both testdepot: (no loaded duplicates) and testdepot (loaded duplicates)

[Keno]

Tested locally with

Add this to the test suite?