Magic mutation fields defined on abstract types are incorrectly imported

[rbalicki2]

If you define a magic mutation field on an abstract type (e.g. Starrable), but use it on a concrete type (e.g. Repository), the Starrable/reader.ts file will not be generated. But, the generated reader artifact where the mutation field is referenced will try to import Starrable/reader instead of Repository/reader.

[rbalicki2]

Fixed by cfad50631f49ef2e31c8510720b4759012152d73