backend: don't run createrepo for srpm directories
    
    This both saves some storage and CPU time on Backend, but it also
    fixes the traceback caused by f61db18a4a2935729 (since that commit
    we did not expect that createrepo might be run against SRPMs dirs
    at all):
    
    File "/usr/share/copr/backend/daemons/worker.py", line 230, in do_job
        mr.on_success_build()
      File "/usr/share/copr/backend/mockremote/__init__.py", line 232, in on_success_build
        self.do_createrepo()
      File "/usr/share/copr/backend/mockremote/__init__.py", line 217, in do_createrepo
        projectname=self.job.project_name,
      File "/usr/share/copr/backend/createrepo.py", line 182, in createrepo
        out_ad = add_appdata(path, username, projectname)
      File "/usr/share/copr/backend/createrepo.py", line 121, in add_appdata
        assert parent_dir.endswith(os.path.join(username, projectname))
    AssertionError