Inject xrefs for externally provided mappings into release version of ontology

[cmungall]

Some xrefs are maintained by ontology eds, some come from outside (e.g. interpro2go)

For external ones, we should inject into the release obo version.

We also want to deprecate URLs like

http:https://geneontology.org/external2go/interpro2go

and have these be PURLs analogous to subsets, imports