Incorrect handling of external references?

Description

I have an OpenAPI 3.0 API file that references external JSON schemas, as seen below:

  /voterReg:
    post:
      externalDocs:
        description: NIST VRI Documentation
        url: https://pages.nist.gov/VoterRecordsInterchange/
      operationId: VoterRegPost
      requestBody:
        description: A VRI Records Request object
        required: true
        content:
          'application/json':
            schema: 
              $ref: "../NIST_VRI_voter_records_interchange.json#/definitions/VRI.VoterRecordsRequest"

This external schema is dereferenced correctly. However, NIST_voter_records_interchange.json references an external schema as well, addr.json, which is not.

	"$ref": "addr.json#/definitions/addr.GeneralAddressClass_type"

For some reason, swagger-codegen expects a file that is the concatenation of the two:

java.lang.RuntimeException: Could not find ../schemas/NIST_VRI_voter_records_interchange.jsonaddr.json on the classpath
        at io.swagger.v3.parser.util.ClasspathHelper.loadFileFromClasspath(ClasspathHelper.java:31)
        at io.swagger.v3.parser.util.RefUtils.readExternalRef(RefUtils.java:153)
        at io.swagger.v3.parser.ResolverCache.loadRef(ResolverCache.java:116)
        at io.swagger.v3.parser.processors.ExternalRefProcessor.processRefToExternalSchema(ExternalRefProcessor.java:47)
(snipped)

Swagger-codegen version

3.0.0-SNAPSHOT

Swagger declaration file content or url

See gist

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i vri.yaml -l php -o ./

Steps to reproduce

Occurs with any language target

Related issues/PRs

Suggest a fix/enhancement

Reproduce the nested-reference case from the linked gist with the Java CLI command, then trace the reported path through RefUtils.readExternalRef, ResolverCache.loadRef, and ExternalRefProcessor.processRefToExternalSchema. Done means an external schema referenced from another external schema resolves as its own file instead of producing a concatenated path error.