Allow overlapping keys in polymorphic queries

[scotttrinh]

Currently if two subtypes contain the same keys, using the e.is function cannot generate the proper EdgeQL query.

Example:

Schema:

abstract type A {
  a: str;
}

type B extending A {
  b: int;
}

type C extending A {
  b: str;
}

EdgeQL:

select A {
  a,
  [is B].b,
  [is C].b
}

Query Builder:

e.select(e.A, () => ({
  a: true,
  ...e.is(e.B, { b: true }),
  ...e.is(e.C, { b: true }),
}));

In the Query Builder case, you end up overriding the e.B property with e.C rather than having both of them end up in the final query data structure.

[jaclarke]

Duplicate of #630