Fix interface not possible type when implementing another interface

[JohnStarich]

Fix interface not possible type when implementing another interface

Some schemas may have interfaces that implement interfaces, like so:

interface Node {
    id: ID!
}

interface Bar implements Node {
    id: ID!
}

type Foo implements Node & Bar {
    id: ID!
}

Node should be a possible type of Bar, just like Node and Bar are possible types of Foo.

[JohnStarich]

This is the same strategy in gqlparser: https://github.com/vektah/gqlparser/blob/41b7913b390ac4278ca7fc766afd586c1e6df819/validator/schema.go#L82-L85