Filtering base interface selections is not type-aware

[gmac]

Another crazy one along the lines of #87, and confirms the issue hypothesized in #91.

The Problem

During final selection filtering in the execution pipeline, type awareness seems to be lost on base interface selections (i.e.: fields selected from an interface without going through a typed fragment). For example, here myInterface is an interface type that offers myUnion as a base field:

query {
  myInterface {
    myUnion {
      ...on Product {
        name
        manufacturer {
          name
        }
      }
    }
  }
}

Stacking these abstract selections atop one another results in the following unsuccessful outcome... while the underlying queries look accurate, the final filtered result is off:

{
  "data": {
    "myInterface": {
      "myUnion": {
        "name": "iPhone",
        "manufacturer": {
          "_bramble_id": "1"
        },
        "_bramble_id": "1",
        "_bramble__typename": "Product"
      }
    }
  }
}

HOWEVER, this issue can be avoided by wrapping the second-level union in a typed fragment. The following selection works perfectly, implying that the problem is specifically with base interface selections:

query {
  myInterface {
    ...on Product {
      myUnion {
        ...on Product {
          name
          manufacturer {
            name
          }
        }
      }
    }
  }
}

Probable cause

I believe this requires a complimenting fix to the changes made in #91. While #91 assured that abstract types always have access to their type information, I believe that the execution pipeline must still be updated to leverage this base type information.

[gmac]

Actual cause

The myInterface field was being treated as a namespace, and the namespace handler block was modifying the original user selection during planning rather than adjusting a copy. This is addressed in #108.

The fact that myInterface was being treated as a namespace is debatably problematic, though not strictly broken. A proposed fix for supporting interfaces with first-class routing is in #107.