Add support for internally scoped functions

[Wernerson]

Great plugin!

However, my code currently wont compile if one of my classes that I want to map is internal because the extension function is generated as public and thus exposes an internal type.
Also I think you have to add mapstruct explicitly as dependency in order for it to work.

I'll take a look at your code tomorrow and maybe even do a pull request, well see :)
But internal support would be great 👍

[driver733]

@Wernerson Thanks for the suggestion! Glad you like the plugin. Yeah, it would great if you could create a PR.