fix(jsonschema,openapi): nested references not resolved

[mtrimolet]

What this PR does / why we need it:

Local references in referenced schemas were resolved relative to top level schema.
Expected behavior is to resolve them relative to referenced schema.

Types of changes.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes.

fixes #15

How Has This Been Tested?

spec2ts/packages/jsonschema/tests/schema-parser.spec.ts

Line 198 in 1d35719

test("should resolve nested references from their own context", async () => {

spec2ts/packages/openapi/tests/openapi-parser.spec.ts

Line 105 in 1d35719

test("should properly resolve nested references", async () => {

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.