Refactoring - Finder no longer extends Index

Composition here seems to be better than inheritance. A `Finder` is not an
`Indexer`.
Refactoring - Finder no longer extends ProjectFinder

Composition here seems to be better than inheritance.
Refactoring - SourceIndexer no longer extends Index

Composition here seems to be better than inheritance. A `SourceIndexer`
is not an `Index`. Rather, a `SourceIndexer` uses an `Index`, which is
why composition is better in this case.