Fix import mappings for service in angular typescript

Created by: rudolficzek

fixes https://github.com/OpenAPITools/openapi-generator/issues/5079 Currently line https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript-angular/api.service.mustache#L12 causes everything to be imported from ../model/models

So for a mapping :

"importMappings": { "MyObject": "@company/mylib" }

actual is:

import { MyObject} from '../model/models';

Expected:

import { MyObject} from '@company/mylib';

Solution exist in typescript-node to handle that. Pr aligns two generators to be symmetric

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