Example usage:
>>> from gentoolkit.helpers import get_cpvs
>>> len(set(get_cpvs()))
- 26065
+ 33518
>>> fn = lambda x: x.startswith('app-portage')
>>> len(set(get_cpvs(fn, include_installed=False)))
- 112
+ 137
@type predicate: function
@param predicate: a function to filter the package list with