Provider structure refactor

[Raffo]

This PR refactors the provider structure by separating them in different folders. I've touched only the providers and none of the logic of the providers themselves. I'm fairly convinced that this is the right structure to allow us to move forward and scale the maintainers as discussed in #1558 .
I will not:

• touch the sources (that will be done in a later PR)
• touch other parts of the code / do further restructuring unless mentioned in the code review.

I am aware that this will put a bit of additional effort on the existing PRs as they'll likely need to rebase those changes, this is why I don't want to touch too much the code.

/cc @linki @njuettner

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[njuettner]

Thanks Raffo, ok for me 👍. I just merged one PR for dnsimple, would you mind fixing it?
@linki you wanna have a look as well 🤗 ?

[Raffo]

Sure, will fix the dnsimple stuff.

EDIT: done.

[njuettner]

/lgtm

[Raffo]

@njuettner I'll wait a couple of days max for @linki in case he wants to further review this, then proceed with merging this as I'd rather have this branch be short lived, given the impact it has on the code structure.

[Raffo]

oh well, the bot was faster 😅