codeintel: extend PackageDependency for dependency search
Created by: tsenart
Context
This commit extends the reposource.PackageDependency
interface with
methods necessary for implementing dependency search (in a follow up PR).
Some methods of the added methods are already implemented by the embedded type as long as it is a pointer, so we change signatures everywhere to deal with pointers rather than values, which should have the added benefit of avoiding a fair bit of unnecessary memory copies.
Part of #28932
Test plan
This intends to be a semantics preserving refactoring which should be covered by existing unit and integration tests.