Librarian-puppet integration

Hey

I was considering working on replacing r10k's own Puppetfile management with `librarian-puppet`. I have been poking around librarian-puppet to see how hard it would be to integrate. From what I can see so far - it doesn't look /too/ bad.

If this were done, it would also close off a number of issues.

- #89 
- #97 
- #92 
- #89 
- #78 
- #47 
- #50 

And maybe more (they are all I could see). Adrien/others, what do you think? It feels this is the right direction to take.

Cheers!

Update: I've been able to replace `r10k puppetfile install` with a native librarian-puppet call which is working quite well.