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

[rbalicki2]

• 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)