Traverse a used type and record all the dependencies we need to keep track of for incremental recompilation.

As a motivating example, given a type T defined as:

type T >: L <: H type L <: A1 type H <: B1 class A1 extends A0 class B1 extends B0

We need to record a dependency on T, L, H, A1, B1. This is necessary because the API representation that ExtractAPI produces for T just refers to the strings "L" and "H", it does not contain their API representation. Therefore, the name hash of T does not change if for example the definition of L changes.

We do not need to keep track of superclasses like A0 and B0 because the API representation of a class (and therefore its name hash) already contains all necessary information on superclasses.

A natural question to ask is: Since traversing all referenced types to find all these names is costly, why not change the API representation produced by ExtractAPI to contain that information? This way the name hash of T would change if any of the types it depends on change, and we would only need to record a dependency on T. Unfortunately there is no simple answer to the question "what does T depend on?" because it depends on the prefix and ExtractAPI does not compute types as seen from every possible prefix, the documentation of ExtractAPI explains why.

The tests in sbt types-in-used-names-a, types-in-used-names-b, as-seen-from-a and as-seen-from-b rely on this.