Everything uses Python 3 nowadays and the python = python3 override is
only needed for ROS 1 rosdep packages. Move this override to
ros1-overlay.nix, and make sure to use python3 explicitly everywhere
else.
It was too ambitious to enable strictDeps all the time, as too many packages
need fixes. Now it is only enabled for Python packages and when cross-compiling.
nixpkgs now requires explicitly enabling or disabling Qt5 wrapping. This commit
disables wrapping by default, which breaks Qt apps that aren't overridden
manually in distro-overlay.nix, but fixes all the Qt libraries that don't need
wrapping. This is a sub-optimal solution, but matches the behavior that was
there before wrapping was made mandatory.
