wmflib: add require_package() from vagrant

It's similar to ensure_packages(), but it's cleaner and faster. It takes a
single package name as an argument and it creates a virtual class that wraps it
and makes it a requirement for the current scope. We can use this to avoid
duplicate def'n errors for packages that may legitimately be specified in the
software stacks of multiple modules / roles.

Change-Id: I155a36c8e650dcc90a479cbcf1bb57259add3633