Fix naming collisions in Schema Generation

[carltongibson]

Closes #4704

WIP DO NOT MERGE

If urls include elements in ['retrieve', 'list', 'create', 'update', 'partial_update', 'destroy'] — I **think** just those — schema generation can fail is a **nested** item clashes with a previously defined Link`.

(We want a dictionary to insert a Link to, but a Link is already there.)

Thus far, just added a couple of failing test case for the reported cases.

[tomchristie]

Okay, so the easiest way to review this would be to comment out in this ticket what the simplest example of a schema collision is. In the shown case, do we have two different links that are both supposed to be included as this?...

{
    "retrieve": Link()
}

If so, what would we expect the schema structure to look like instead?

I suppose we could do something like fall back to...

{
    "retrieve": Link()
    "test_retrieve": Link()
}

Tho it's not obvious how we'd make that work reliably.

Alternatively, could we raise a clear exception explaining what the issue is, and document this as a constraint? (allow the user to either renamed the URL or customize the structure generation?)

[carltongibson]

I've adjusted insert_into to raise a ValueError with a helpful message here. It's not perfect but:

... it's not obvious how we'd make that work reliably.

I'd be happy to see a follow-up PR here.

[tomchristie]

I think we should role with this approach for now, yup.

(And yes, we can still follow-up on it later)