Relay IDs and scalar types should be opaque types in flow

Currently the flow types generated by relay use `string` type to represent the ID. The type for the ID field should be a flow opaque type so that users on cannot pass it arbitrary strings as IDs.

This behaviour should be extended for custom scalar types. I.e. Every scalar type should be a unique opaque type.

```
export type RestrictedPartyHitsPanelRestrictedPartyHitScanForCompanyMutationResponse = {|
+RestrictedPartyHitScanForCompany: ?{|
+company: ?{|
+id: string, // CAN THIS BE A an opaque type
....
```

No file or test path is named. Start at Relay's Flow type-generation entry point and trace how the shown ID and custom scalar fields are emitted; done means generated IDs and each scalar use distinct opaque types, with tests confirming arbitrary strings are rejected.

Written by the indexing model from the issue text.