Use a deepcopy when assigning list_pkgs results to __context__

This fixes a problem that Dave found where a simple assignment caused
the data in the return dict to be linked to __context__['pkg.list_pkgs']
due to a shallow copy, and thus the call to pkg_resource.stringify was
modifying the data in __context__['pkg.list_pkgs']. Dave will be pushing
a commit later on that fixes this in win_pkg.py, this commit fixes it in
the rest.