fix(gatsby): support union types for one-to-one ___NODE relations

[vladar]

Description

This PR should make the processing of ___NODE convenience fields consistent for one-to-one and one-to-many relations (currently one-to-many works as expected while one-to-one doesn't).

Take this example with one-to-one relation:

createNode({
  internal: { type: `MyType` },
  related___NODE: `FooTypeNode1`,
})
createNode({
  internal: { type: `MyType` },
  related___NODE: `BarTypeNode1`,
})

The expected inference result is a union type:

type MyType {
  related: FooBar
}

union FooBar = Foo | Bar

The actual result is a first registered object type:

type MyType {
  related: Foo
}

The same example for one-to-many relation produces an expected result:

createNode({
  internal: { type: `MyType` },
  related___NODE: [`FooTypeNode1`],
})
createNode({
  internal: { type: `MyType` },
  related___NODE: [`BarTypeNode1`],
})

Results in:

type MyType {
  related: [FooBar]
}

union FooBar = Foo | Bar

Related Issues

Fixes #10090

[pvdz]

Looks fine to me.

How big are these clusters of nodes? Because if we Object.keys() them at every turn, or they are big with random key names, we should really consider to use Maps here. I suspect that'll make a big difference.

Approving since that's not in the scope of this PR. Food for thought.