External references are not dereferenced in header->schema

[radhus]

When generating server response models, external package imports are not being set on the referenced header types when their schema specifications are also refs. Similar, but not equal, to issue #1087.

I took a quick look at the code but couldn't very easily figure out where to fix it. Might have time to look over it in more detail at a later time and see if I can produce a fix myself.

Reproducing steps

Building on the import-mapping/multipackages examples.

Full example committed here: https://github.com/radhus/oapi-codegen/commit/695a09a9c2907cb064344dc4fc2cd7be35c16d02

Add a header and schema in common/api.yml:

components:
  schemas:
    [...]
    ETagSchema:
      type: string
      description: RFC 7232

  headers:
    ETag:
      description: Custom header
      schema:
        $ref: "#/components/schemas/ETagSchema"

Use header in admin/api.yml:

paths:
  /admin/user/{id}:
    get:
      [...]
      responses:
        200:
          [...]
          headers:
            ETag:
              $ref: '../common/api.yaml#/components/headers/ETag'

Struct with wrong type

Regenerate with strict-server: true to get the response structs generated. Look at admin/server.gen.go

type GetUserById200ResponseHeaders struct {
	ETag ETagSchema
}

Expected result

The schema should be referenced from the other package:

type GetUserById200ResponseHeaders struct {
	ETag externalRef0.ETagSchema
}

[LarsStegman]

Maybe related #2088 ?