git_tag: Fix checking out of workspace when using submodules

Submodule URLs need to be marked as non-primary.

Otherwise this assertion is thrown:

    Traceback (most recent call last):
      File "/home/user/.local/bin/bst", line 10, in <module>
        sys.exit(cli())
      File "/usr/lib/python3/dist-packages/click/core.py", line 764, in __call__
        return self.main(*args, **kwargs)
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/_frontend/cli.py", line 174, in override_main
        standalone_mode=standalone_mode, **extra)
      File "/usr/lib/python3/dist-packages/click/core.py", line 717, in main
        rv = self.invoke(ctx)
      File "/usr/lib/python3/dist-packages/click/core.py", line 1137, in invoke
        return _process_result(sub_ctx.command.invoke(sub_ctx))
      File "/usr/lib/python3/dist-packages/click/core.py", line 1137, in invoke
        return _process_result(sub_ctx.command.invoke(sub_ctx))
      File "/usr/lib/python3/dist-packages/click/core.py", line 956, in invoke
        return ctx.invoke(self.callback, **ctx.params)
      File "/usr/lib/python3/dist-packages/click/core.py", line 555, in invoke
        return callback(*args, **kwargs)
      File "/usr/lib/python3/dist-packages/click/decorators.py", line 27, in new_func
        return f(get_current_context().obj, *args, **kwargs)
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/_frontend/cli.py", line 719, in workspace_open
        force=force)
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/_stream.py", line 525, in workspace_open
        target._open_workspace()
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/element.py", line 1858, in _open_workspace
        source._init_workspace(temp)
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/source.py", line 710, in _init_workspace
        self.init_workspace(directory)
      File "/home/user/.local/lib/python3.7/site-packages/bst_external/sources/git_tag.py", line 686, in init_workspace
        mirror.init_workspace(directory)
      File "/home/user/.local/lib/python3.7/site-packages/bst_external/sources/git_tag.py", line 445, in init_workspace
        url = self.source.translate_url(self.url)
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/source.py", line 547, in translate_url
        self.mark_download_url(url, primary=primary)
      File "/home/user/.local/lib/python3.7/site-packages/buildstream/source.py", line 594, in mark_download_url
        "Primary URL marked twice with different URLs"
    AssertionError: Primary URL marked twice with different URLs