Make use of 'repository' field

[jamesaoverton]

The current code uses the 'tracker' field to guess at the GitHub repository:

https://github.com/cthoyt/obo-community-health/blob/main/build.py#L121

Obviously this works pretty well. It doesn't work for OBI, which uses a PURL for its tracker.

The OBO metadata schema has a 'repository' field, which has better semantics for this purpose:

https://github.com/OBOFoundry/OBOFoundry.github.io/blob/master/util/schema/registry_schema.json#L368

Note that 'tracker' is required metadata, and has been manually updated, while 'repository' is not required and is not filled out very often. So I understand why 'tracker' is being used.

My suggestion is to look for 'repository' first, then fallback to 'tracker'.

See also OBOFoundry/OBOFoundry.github.io#848