Updates to support scratch module builds
    - Add scratch module build sub-commands and updates for handling custom modulemd file and SRPMs.
    - Add tests for scratch module builds and adjust existing tests for new 'scratch' option.
    - Allow custom SRPMs with local module builds for consistency with module scratch builds.
    
    Signed-off-by: Merlin Mathesius <mmathesi@redhat.com>