Support multi-family repositories in crater

[rsheeter]

To recruit more families into crater (https://googlefonts.github.io/fontc_crater/) add the ability to specify where config.yaml lives to METADATA.pb.

For example, Rubik Pixels doesn't compile identical, seems to have metadata (https://github.com/google/fonts/blob/main/ofl/rubikpixels/METADATA.pb), and doesn't appear to be represented on fontc crater. I think it's because it doesn't specify which config.yaml to use within the target repo.

It looks to me like we have no field to specify a path to config.yaml, perhaps we need to add one in https://github.com/googlefonts/gftools/blob/main/Lib/gftools/fonts_public.proto#L80 and use it when identifying sources (https://github.com/googlefonts/google-fonts-sources).

[rsheeter]

We also need to look for config.yaml as a sibling of METADATA.pb, as used by bigshoulders variants (per @simoncozens). Ex https://github.com/google/fonts/tree/main/ofl/bigshoulderstext