Fix nested fragments on interfaces (when a directive is present)

[trevor-scheer]

The following query:

query {
  i {
    _id
    ... on I2 {
      ... on I2 @test {
        id
      }
    }
  }
}

Blows up with the following error:

Cannot add fragment of condition "I2" (runtimes: [T1,T2]) to parent type "T1" (runtimes: T1)

When the @test directive is removed, we collapse the inline fragments and this error doesn't occur, but we can't do that when a directive application is present.

This seems to be due to the way that the query planner handles interface conditions, by expanding them into their runtime types. My understanding right now is roughly this:

Expanding the above query in theory looks something like:

query {
  i {
    _id
    ... on T1 {
      ... on I2 @test {
        id
      }
    }
    ... on T2 {
      ... on I2 @test {
        id
      }
    }
  }
}

...but this T2 as parent type of an I2 is triggering the error since canRebaseOn is returning false.

Minimal reproduction of #2862

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[dariuszkuc]

Superseded by #2875