gather: Don't trim prepopulate package
    
    If a package is listed in prepopulate file, it should not be removed
    from the variant during trimming.
    
    Example situation:
    1. Initial release ships package in Server-optional.
    2. In next release prepopulate is used to keep packages in the same
       locations.
    3. An addon starts pulling in the package. Trimming removes it from the
       original location.
    
    This results in consumers being surprised about packages going missing.
    Just because a package is available in an addon is not an excuse for it
    to disappear (and possibly break dependencies).
    
    The solution is to not trim anything pulled in due to prepopulate. As an
    end result, the same package may be included in multiple variants.
    
    JIRA: COMPOSE-2948
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>