fix: don't parse non-exported fields in structs

[Sadzeih]

This fixes a bug where having private fields would add them to the struct as "composed" fields.

This fixes the issue by not adding private fields.

Example

// @openapi:schema
type animal struct {
	species string
}

// Dog struct
// @openapi:schema
type Dog struct {
	Pet
	animal
	otherpackage.Data

	Name    string `json:"name"`
	weight  int
	friends []animal
}

⬇️

    Dog:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - $ref: '#/components/schemas/animal'
      - $ref: '#/components/schemas/WeirdCustomName'
      - type: object
        properties:
          name:
            type: string
    animal:
      type: object

[denouche]

Hello,
I'm not sure to understand, the example you give is before or after?
Can you give the before/after to compare?
By the way you need to rebase your branch, I just merged your other PR :)
Thanks!