[BUG] typescript-fetch doesn't work with external libraries when using importMappings

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When I have an import mapping set up for an external library, typescript-fetch doesn't handle it correctly. I checked using typescript-angular, and that handles it correctly.

The use case here is that the OpenAPI spec needs to be able to reference model class names that exist in another external TypeScript package. In my example, @somecompany/package is a package that contains TypeScript types and interfaces for all of our models, and the example-api-client ideally can reference that package. It would be a dependency of the client.

Workaround: I haven't tried this yet, but I assume I can use templates to put the correct import mappings in place.

openapi-generator version

openapi-generator-cli 6.0.1
  commit : 9e412ed
  built  : 2022-07-03T16:21:27Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

OpenAPI schema (schema.json):
https://gist.github.com/robbieaverill/63d28f9e89ec6184af333fb6a8fb5f30

Config file (codegen.json):

{
  "npmName": "example-api-client",
  "importMappings": {
    "SanitizedAgent": "@somecompany/package/types/agents"
  },
  "validateSpec": false
}

Generation Details

Steps to reproduce

openapi-generator generate -i schema.json -g typescript-fetch -c codegen.json -o client

Inspect client/src/apis/DefaultApi.ts, look at the imports.

Result:

import * as runtime from '../runtime';
import type {
  @somecompany/package/types/agents,
} from '../models';
import {
    @somecompany/package/types/agentsFromJSON,
    @somecompany/package/types/agentsToJSON,
} from '../models';

Expected result:

import * as runtime from '../runtime';

import { SanitizedAgent } from '@somecompany/package/types/agents';
import {
    SanitizedAgentFromJSON,
    SanitizedAgentToJSON,
} from '../models';

If I run the same generation command using typescript-angular, I get the correct import mapping:

openapi-generator generate -i schema.json -g typescript-angular -c codegen.json -o angularclient

Inspect angularclient/src/api/default.service.ts, look at the imports.

I see the expected import mapping from my external package:

// @ts-ignore
import { SanitizedAgent } from '@somecompany/package/types/agents';

Related issues/PRs

• #5079
• https://github.com/OpenAPITools/openapi-generator/pull/10644 Adds support for this feature to typescript-angular

Suggest a fix

Perhaps port the same changes from the PR above into typescript-fetch

Reproduce with schema.json, codegen.json, and the openapi-generator generate command for typescript-fetch, then inspect client/src/apis/DefaultApi.ts. Compare the typescript-angular handling and the changes in PR #10644; done means the generated imports use SanitizedAgent from the external package while the serialization helpers remain valid.