(main) Add read accessor for importer.

This to help debugging.
Adjust tests to not include spec helper at rspec config time.
Fix type_loader_spec test mocking the wrong object.

(The mocking in type_loader_spec still does not clean up ok.
If executed before
spec/unit/pops/impl/transform/transform_conditionals.rb, the second fail
due to getting wrong instance of AstTransformer !!??)