Summary: `transformModulePath` used to be an optional string, because `ConfigT` allowed for an optional `getTransformModulePath` method. Effectively, we required it to be present. This builds on top of the cleanups around `ConfigT` and gets rid of the flow error suppressions
Reviewed By: jeanlauliac
Differential Revision: D5037466
fbshipit-source-id: bc5c9cbc566e7aa74e7f6397e69fa87cdac7bc00