The pkg.Interface style is modeled after heap.Interface. Generally, I
find it helpful for interfaces that have many implementations. It
provides clear distinction between the generic interface and the |n|
implementations that implement it (which may be interface types
themselves).

For clients who cannot keep the repo name, one can imagine that the most
likely rename is `ipfsrepo`. In that case, `ipfsrepo.Interface` remains
meaningful.

This is low-pri so it doesn't matter than much. But for the record, the
repo.Interface feels appropriate in this use-case.