qt: make package names of "full" variants appear as such

Currently there is no distinction in the package names of Qt packages
whether they are "full" or not (i.e. coming from a qt5Full or qt5
attribute).

Currently:
  $ nix-env -f. -qaP | grep "^qt[45]"
  qt48Full   qt-4.8.6
  qt5Full    qt-5.2.1
  qt5        qt-5.2.1
  qt53       qt-5.3.1
  qt53Full   qt-5.3.1
  [...]

With this change:
  $ nix-env -f. -qaP | grep "^qt[45]"
  qt5        qt-5.2.1
  qt53       qt-5.3.1
  qt48Full   qt-full-4.8.6
  qt5Full    qt-full-5.2.1
  qt53Full   qt-full-5.3.1
  [...]

NOTE: This only touches the "full" variants (which are basically unreferenced in nixpkgs), so it will not cause a massive rebuild.