Allow name and package_name to be different to avoid CHEF-3694 warnings

This makes it possible avoid CHEF-3694 warnings using code like this:

python_pip "some_lib /ve1" do
  package_name "some_lib"
  virtualenv "/ve1"
end

python_pip "some_lib /ve2" do
  package_name "some_lib"
  virtualenv "/ve2"
end