[bug] not importing related .grapgql files during codegen

[alvis]

With 2.0, related .graphql files are no longer imported into the schema when we run graphql codegen. It is due to a bug that prisma-binding do not handle any import syntax at the moment. See line 73:
https://github.com/prismagraphql/prisma-binding/blob/ef7fc575a8a5473db2f48574c488c71ca181bf9f/src/bin.ts#L71-L86

To fix it, we should wrap it with importSchema from graphql-import, i.e.

return buildSchema(importSchema(input))

[schickling]

Thanks a lot for reporting this @alvis. @timsuchanek will shortly look into this and merge your PR