Gracefully handle duplicate schema elements

[Tomboyo]

As a user, when I try to compile an OAS document that has duplicate elements due to author error, Lily should warn me about the error (and tell me which generated types are impacted), but continue generating classes so that I can work with as much of the OAS that is well-formed as possible rather than be completely blocked.

For example:

openapi: 3.0.2
  components:
    schemas:
      Foo: ...
      Foo: ... # A duplicate component

This should also be the case when Lily decides to name an anonymous type in such a way that it collides with another type. Ideally this is logged at a more severe level than usual since it's a design flaw on our part, if possible.