Find added stubs in daemon when using --follow-imports=skip. Previously
it was often necessary to restart the daemon to find new stubs. This was
inconsistent, as the initial build did follow imports to stubs but incremental
runs didn't (at least reliably).

Add an optimization that avoids checking modules within top-level packages
that clearly don't exist.

This still can regress performance in large codebases somewhat significantly.
I'll create follow-up PRs that make the regression less bad.

Also mark follow_imports_for_stubs as unsupported in the daemon. I don't
think that it worked reliably before, and now it works even less well.