Rework spec composition to make sure to use custom names for types, also adds interface{} and external package handling

[Sadzeih]

This PR reworks the composition of the spec to take into account #30 and #31.

This allows you to do this:

// @openapi:schema
type Response struct {
    Data external.MoreData `json:"data"`
}

:arrow_down:

Response:
    type: object
    properties:
        data:
            $ref: '#/components/schemas/MoreData'

and

// @openapi:schema:CustomName
type RegularType struct {
 // properties
}

// openapi:schema
type Response {
    Data RegularType `json:"data"`
}

:arrow_down:

Response:
    type: object
    properties:
        data:
            $ref: '#/components/schemas/CustomName'
CustomName:
    type: object
    properties:
        # properties

[Sadzeih]

Sorry, this PR is actually now a lot more than what the title says. It also found a better way to do #29 and adds some support for Redoc x-groupTags

[denouche]

Wow, this is a great PR, thank you! Sorry for the response time. Can you rebase your branch please?

[Sadzeih]

No worries about the response time, we've been using my fork in the meantime. What do you want me to rebase on? I'm up to date with master.

[denouche]

I cannot merge it, it tells me the branch has conflicts:

[denouche]

Indeed your branch is just ahead of 13 commits from master, there should not be conflicting... I will merge it manually.

[denouche]

Merged. Thank you for your contributions!