[display] consider abstracts as implementations of their underlying type

I think it would make a lot of sense it abstracts were included in `display/implementation`: an `abstract A(B)` is an implementation of `B`.

Start by locating the compiler code that computes `display/implementation` and the handling of abstract types. Confirm how an abstract `A(B)` is represented, then add coverage showing that A is reported as an implementation of B; done means the display result includes this relationship without breaking existing cases.

Written by the indexing model from the issue text.