Depend on puppetlabs-git to manage git package

This is a suggestion to switch to https://github.com/puppetlabs/puppetlabs-git.

I'm using some modules that rely on puppetlabs-git. It produces a conflict with etckeeper, depending on the inclusion order.

Although etckeeper does it fine with the `defined()` check, I think that leveraging puppetlabs-git is better from a _single responsibility principle_ perspective.