Create utilities for module generators

Summary:
There are two operations we do in every NativeModule generator:
- We convert the `SchemaType` into a map of NativeModule schemas
- If the type-annotation is a TypeAliasTypeAnnotation, we resolve it by doing a lookup on the NativeModuleAliasMap. This is usually followed by an invariant to assert that the lookup didn't fail.

Both procedures have been translated into utilities for use across our generators. I also deleted `getTypeAliasTypeAnnotation` which will no longer be used.

Changelog: [Internal]

Reviewed By: hramos

Differential Revision: D23667249

fbshipit-source-id: 4e34078980e2caa4daed77c38b1168bfc161c31c