Move provider implementations under their related projects.

[Eric Snow]

Perhaps we should move the implementation of some providers over to
the repos for the projects with which the providers interact (and then
just import them in providers/all).  Then those providers would be
more likely to stay up-to-date in the face of changes in the project,
particularly backward-incompatible ones.  For example:

* provider/maas -> github.com/juju/gomaasapi/maasprovider
* provider/lxd -> github.com/lxc/lxd/lxdprovider
* ...

or something like that.  It's not a perfect solution nor a complete
one, but it should help.

-eric