Rename `InterpreterSession.self` to `InterpreterSession.importer`

[PaliC]

We should rename this to make this more clear that this is an importer not the actual interpreter. At the moment, it is pretty easy to confuse this variable as the interpreter itself, and given it's assigned from createOrGetPackageImporterFromContainerFile and the typing requires it to be a Obj. Instead, let's have a more descriptive name.