This essentially rewrites default.nix.
First off, we start using the magic of scopes and create a scope for the
wordpress packages. This scope is still extensible but also contains
utilitiy functions now, namely mkWordpressDerivation,
mkOfficialWordpressDerivation, filterWPString, and fetchWordpress.
This way, we are exposing almost all functionality to third-party software which can extend the package set and use the existing functionality.
mkWordpressDerivation now properly calls the nixpkgs hooks and adds
the package type into the pname.
We also add extend functions to all package groups (meaning you can
now do plugins.extend instead of //ing it and add some overrides
from helsinki here.