WIP: Fix expand remote

[fredbi]

circular $ref expansion: fixed edge cases

• fixes Expander: expanded document is not self-contained with some circular $ref patterns #94

• now expanded $ref's are always contained in the resulting document.
  All circular $ref that used to resolve to a remote $ref now resolve
  as a json pointer inside the expanded document. Pointer resolution
  prefers pointers to definitions.

• added additional test case for remote cyclical $ref, from azure API

• schema IDs are removed from the expanded spec: schemas expanded from some schema ID reference
  now refer to the new expanded root document.

• circular IDs are resolved against the corresponding root document.

NOTE(1): uncovered pre-existing issue with nested schema ID involving cyclical references.
This case remains unsupported and is illustrated by test case: circular_test.go#L198 ("withID")

NOTE(2): pre-existing issue with non-deterministic expansion remains unsolved,
although the election of the replacing pointer inside the root document
somewhat reduces the scope of this problem.

This case remains illustrated by a minimal test case: circular_test.go#L46 ("minimal"),
which expands correctly, but with changing results.

NOTE(3): notice that expansion is still not an idempotent transform, in the presence
of cyclical $ref's: another run on an expanded spec with remaining cyclical $ref
will expand further down and detect again the cycle.

The result remains functionally correct, as illustrated by test case: circular_test.go#L168 ("CircularID").
Notice that this test case reproduces a validation fixture from jsonschema test (passed by go-openapi/validate).

Signed-off-by: Frederic BIDON fredbi@yahoo.com

[codecov]

Codecov Report

Merging #137 (e606f6b) into master (efe8fb3) will increase coverage by 0.90%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
+ Coverage   62.18%   63.08%   +0.90%     
==========================================
  Files          27       27              
  Lines        2052     2086      +34     
==========================================
+ Hits         1276     1316      +40     
+ Misses        605      592      -13     
- Partials      171      178       +7     

Impacted Files	Coverage Δ
resolver.go	56.86% <0.00%> (ø)
expander.go	77.77% <79.16%> (-0.80%)	⬇️
schema_loader.go	89.41% <89.28%> (+1.17%)	⬆️
normalizer.go	85.71% <92.85%> (+2.13%)	⬆️
swagger.go	66.29% <0.00%> (+1.12%)	⬆️
ref.go	43.24% <0.00%> (+2.70%)	⬆️
parameter.go	22.64% <0.00%> (+3.77%)	⬆️

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