Allow foreign key to use an existing index

[WarmBooter]

If you have a PK field that is also a FK field, you end up with two identical indexes. It would be nice to extend the creation of a foreign key to identify and use possible existing indexes instead of creating a new identical one.

[livius2]

During implementation, ensure it's possible to drop the primary key (PK), even if a foreign key (FK) relies on its index.