CB-5299 Speed up prepare by using plugman's new reapply_global_munge()

This is based on reapply_global_munge() from plugman's config-changes.
This function loads the global munge from platform json and applies all of it.
The munge is not re-generated from plugin.xml files as it was previously done.
CB-5647: Remove .staging dir.

Based on plugman changed that moved the <asset> copying stage from install to
prepare.