Dependency checks

This is an initial attempt at installing required package "depends" and "build_depends" with Puppet. Using Puppet's provider to do this means the hard work is done for us by the Puppet folks - no OS/platform detection required, which keeps things nice and simple.
Notes:

* It can't handle the version comparison forms (<<, >>, <=, >=) because Puppet can't handle them; we skip them and print a warning message.
* Handling an "or" string (packages separated with a pipe) isn't possible because we can't assume what the user wants, so we skip them and print a warning message.