Motivation for this change

@cillianderoiste In addition to updating, this overrides possibly pinned versions from those available in the current environment.

If you remember, buildout-nix differs from the original buildout in that it prefers "system packages" over buildout packages: this allows to use hard-to-build python packages from nixpkgs with buildout and also running buildout with only nix-dependencies. My only issue has been the need of manually unpinning the conflicting versions.

With these changes, I can simply

$ nix-shell -p pythonPackages.ldap pythonPackages.lxml pythonPackages.pillow pythonPackages.pyyaml pythonPackages.python_magic pythonPackages.zc_buildout_nix --run buildout-nix

to get a complex Plone buildout run without needing to think about the pinned versions (in this case still installing most of the packages directly from PyPI using buildout).

Nix-pinned packages can be seen using buildout annote

$ buildout-nix annotate|grep -i pillow
Pillow= 3.4.2
     /nix/store/g4p8gi7ygwkcyfj0iqvm4kzq12dvi8af-python2.7-Pillow-3.4.2/lib/python2.7/site-packages

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.