When we created metadata references inside of VisualStudioProject,
we ensured we created them with a file watcher so we would know about
changes. When we created a metadata reference after we converted a
project reference back to a metadata reference that was originally
a metadata reference, we didn't do so since that conversion happens
in VisualStudioWorkspaceImpl itself and it couldn't access the file
watcher.

This fixes this. The majority of the work here is just moving code
around. Basically the watchers were per-project, which meant that
it'd be forbidden to call into them the Workspace code because
that would violate the lock hierarchy. To fix this, I move the file
watching logic into it's own little type with a lock, and since no
calls are made back into the workspace under that lock, it removes
the deadlock concerns.

Fixes dotnet/roslyn#32554.