Correctly resolve externalRefs that point to the root path - #1963

Upon further inspection, this only fixes a portion of the failing cases. I've observed two so far--what I'll call "A-B-A," where schemas in B reference schemas in A, and A is the top-level document, but I've discovered a new one--what I'll call "A-B-C-B," where schemas in A reference schemas in B, and schemas in C reference schemas in B. An example schema is given below (not tested yet):

foo.yaml

components:
  schemas:
    Group:
      anyOf:
      - $ref: ./bar.yaml#idRef
      - $ref: ./baz.yaml#Group_v1
<...>

bar.yaml:

components:
  schemas:
    idRef:
      format: uri-reference
<...>

baz.yaml:

components:
  schemas:
    Group_v1:
      properties:
        id:
          $ref: "./bar.yaml#idRef"
<...>

I believe this fixes all the issues that I have observed. I have not been able to reproduce the "A-B-C-B" case since I originally saw it, so it's entirely possible that was a fluke. I believe this PR is ready for review/merge.

Hi @AmateurECE!
Thank you for creating this PR :) the core idea was useful and helped clarify the right approach for #1961.

We are planning to merge #2383 because it implements that behavior against the current resolver architecture. I have also extended to use URI-based reference identity rather than filesystem Path comparison. That keeps it aligned with the newer normalization work.