Fix bug in oneOf not propagating external references

[technicianted]

This PR fixes a bug in not properly propagating external references when merging schemas of allOf.

If oneOf is used with a type with external reference that itself has a local reference, the generated struct does not have the external reference.

Repro:

schemas:
    ContainerAny:
      allOf:
        - $ref: "sub.yaml#/components/schemas/Inner"
        - type: object
          properties:
            Outer:
              type: string

inner.yaml:

components:
  schemas:
    DeepInner:
      type: object
      properties:
        DeepInner1:
          type: string
    Inner:
      type: object
      properties:
        Inner1:
          type: string
        Inner2:
          $ref: "#/components/schemas/DeepInner"

Generated code with external references not propagated:

// ContainerAny defines model for ContainerAny.
type ContainerAny struct {
	Inner1 *string    `json:"Inner1,omitempty"`
        // this is incorrect as it is missing the package reference
	Inner2 *DeepInner `json:"Inner2,omitempty"`
	Outer  *string    `json:"Outer,omitempty"`
}

With fix:

// ContainerAny defines model for ContainerAny.
type ContainerAny struct {
	Inner1 *string                 `json:"Inner1,omitempty"`
	Inner2 *externalRef0.DeepInner `json:"Inner2,omitempty"`
	Outer  *string                 `json:"Outer,omitempty"`
}