Revert addition of :downstream profile.

We need to revisit this in a way that doesn't involve hard-coding
profile names into the jar and pom tasks. The question of whether the
profile should be visible to downstream consumers should be determined
by data that's part of the profile (or metadata).
Call load-plugins twice to fix chicken/egg problem.

Need to do this in order to support loading profiles from plugins but
also loading plugins from the :user and :system profiles.

fixes #1679
Use plugin name for reading profiles for now.

This is for consistency with reading middleware/hooks. In the future it
might be nice to honor the group-id as well.