[BUG] Import Mapping seems to not work with typescript-angular

Created by: filipvh

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

I used importMapping in combination with the typescript-angular generator. Here is a redacted config file:

{
  "npmName": "the-package-name",
  "npmVersion": "1.8.0",
  "ngVersion": "1.7.0",
  "modelFileSuffix": ".type",
  "npmRepository" : "http://domain/is/redacted",
  "snapshot" : false,
  "importMappings": {
        "EnumValue":"some-other-package"
  }
}

openapi-generator version

4.0.1

What happens?

• The 'EnumValue' is not generated (is missing from model folder) => OK
• In an other type/interface EnumValue is still refferenced locally => NOK

import { EnumValue } from './enumValue.type';

What was expected?

• The 'EnumValue' is not generated (is missing from model folder)
• In an other type/interface EnumValue refferences the external package

import { EnumValue } from 'some-other-package';

Command line used for generation

generate -i model.yaml -g typescript-angular -c config.json -o out