No validation that an @refetchable queryName is not the name of a fragment

* If a @refetchable fragment has a queryName which is the name of an existing fragment (say `MyFragment`), then `__generated__/MyFragment.graphql` will be the `ReaderFragment`, and not the `ConcreteRequest`.
* If a persisted queries json file is created (and presumably if the query is persisted to the backend?), then the persisted "query" is actually just the fragments.
* Relay should enforce that `queryName` is globally unique. (Presumably there can be collisions with other query names, too, didn't check)