Eliminate the lingering association between macro crates and plugins (internals)

After #22026 they are barely related, but the internals do not reflect this fact.  This causes problems like #22197 and is generally confusing.

All we should do for a plugin is find the appropriate dylib file (which might not even be the one you'd want to link), find one metadata entry, and hand that off to the existing loading & registrar code.
