Fix: Ensure external refs are propagated to generated code

As noted in oapi-codegen#1378, there are cases where a complex set of `$ref`s
between multiple files can lead to broken generated code, which does not
correctly import the package that has been prepared for the external
reference.

We can handle this by looking up any references, where there is a `.Ref`
passed into the type, and then iterate through relevant children.

This requires we handle the updating in-place for these by using a bit
of pointer + indexing fun.

This also adds a relevant test case to validate the fix.

Closes oapi-codegen#1378.

internal/test/issues/issue-1378/bionicle.yaml

@@ -0,0 +1,43 @@
+openapi: 3.0.1
+
+info:
+  title: Test
+  description: Test
+  version: 1.0.0
+
+paths:
+  /bionicle/{name}:
+    get:
+      parameters:
+        - $ref: "#/components/parameters/bionicleName"
+      responses:
+        '200':
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Bionicle'
+        '400':
+          content:
+            application/json:
+              schema:
+                oneOf:
+                  - $ref: '#/components/schemas/Bionicle'
+
+components:
+  parameters:
+    bionicleName:
+      name: name
+      in: path
+      description: Name of the character
+      required: true
+      schema:
+        type: string
+
+  schemas:
+    Bionicle:
+      type: object
+      properties:
+        name:
+          type: string
+      required:
+        - name