Add a public API entrypoint for the "oracle" (go to definition) feature

[anderseknert]

We use this in the Regal language server to provide the "go to definition" feature, but since the oracle package is internal, we're currently using a copy-pasted (and slightly modified / improved) version in Regal. If we exposed this feature in OPA via a public package, we'd be able to avoid maintaining this in more than one place, and improvements would benefit both projects.