Structs are not generated when partials intended to add to them exist in the project

[AArnott]

For #78 we added the partial modifier to structs to support folks adding members to them in their own code. But the scenario is still broken because as soon as the user adds their partial struct to complement the projected one, the projected one disappears because we suppress generation when a type already exists.

We should consider whether the existing type we find is in the same compilation and is partial. If so, we should go ahead and generate the struct.