As oras-credentials-go has now reached a fairly stable state, we can proceed with merging it back into oras-go. This will allows us:

1. Reduce maintenance cost
2. Include credentials package in oras-go as an incremental feature with no dependency penalty
3. Increase visibility of oras-credentials-go as a package in oras-go

See oras-project/oras-credentials-go#80 for more details.