Improve mapping compilation results to file URIs

[simerplaha]

Previously, Ast.GlobalDefinition did not contain a SourceIndex, so compilation results were mapped back to their respective file URIs based on the assumption that all type definitions have unique identifiers.

Now, Ast.GlobalDefinition's SourceIndex.fileURI can be used. This fileURI is still an optional type, so reporting compilation should fallback to using the above assumption when fileURI is None.