"Find implementations" is not working for Scala classes and traits

[olafurpg]

Currently, "Find implementation" on Scala traits and super classes is not working as expected. This is because we don't index SymbolInformation.signature, which semanticdb-scalac emits for Scala classes/traits to document the extends ... section.

Steps to reproduce:

• Do "Find implementations" here https://sourcegraph.com/github.com/sourcegraph/scip-java/-/blob/tests/minimized-scala/src/main/scala/minimized/Issue413.scala?L3
• Observe that the "Implementations" section is empty