I noticed in the SDL rendering output that a macro directive that did not
specify the `on` clause would render invalid SDL.

E.g.
```elixir
    directive :foo do
      arg :baz, :string
    end
```

Would render in SDL
```
directive @foo(baz: String) on
```

This PR adds a phase to the schema pipeline to check whether
the directive locations are set. I also check whether the locations
themselves are valid.