Explicitly use python3, move python override to ROS 1

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.

distros/ament-cmake-core-setup-hook/default.nix

@@ -1,6 +1,6 @@
-{ makeSetupHook, python }:
+{ makeSetupHook, python3 }:
 
 makeSetupHook {
   name = "ament-cmake-core-setup-hook";
-  substitutions.python_executable = python.pythonOnBuildForHost.interpreter;
+  substitutions.python_executable = python3.pythonOnBuildForHost.interpreter;
 } ./setup-hook.sh

distros/build-ros-package/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, pythonPackages, rosDistro, rosVersion }:
+{ stdenv, lib, python3Packages, rosDistro, rosVersion }:
 { buildType ? "catkin"
   # Too difficult to fix all the problems with the tests in each package
 , doCheck ? false
@@ -17,7 +17,7 @@
 , ...
 }@args:
 
-(if buildType == "ament_python" then pythonPackages.buildPythonPackage
+(if buildType == "ament_python" then python3Packages.buildPythonPackage
 else stdenv.mkDerivation) (args // {
   inherit doCheck dontWrapQtApps separateDebugInfo;
 
@@ -41,7 +41,7 @@ else stdenv.mkDerivation) (args // {
   # https://github.com/colcon/colcon-core/blob/0.12.1/colcon_core/task/python/build.py#L84
   format = "other";
 
-  nativeBuildInputs = nativeBuildInputs ++ [ pythonPackages.setuptools ];
+  nativeBuildInputs = nativeBuildInputs ++ [ python3Packages.setuptools ];
 
   buildPhase = ''
     runHook preBuild
@@ -54,16 +54,16 @@ else stdenv.mkDerivation) (args // {
   installPhase = ''
     runHook preInstall
 
-    mkdir -p "$out/${pythonPackages.python.sitePackages}"
-    export PYTHONPATH="$out/${pythonPackages.python.sitePackages}:$PYTHONPATH"
+    mkdir -p "$out/${python3Packages.python.sitePackages}"
+    export PYTHONPATH="$out/${python3Packages.python.sitePackages}:$PYTHONPATH"
     python setup.py install --prefix="$out" --single-version-externally-managed --record /dev/null
 
     runHook postInstall
   '';
 
   postFixup = ''
     ${postFixup}
-    find "$out/lib" -mindepth 1 -maxdepth 1 -type d ! -name '${pythonPackages.python.libPrefix}' -print0 | while read -d "" libpkgdir; do
+    find "$out/lib" -mindepth 1 -maxdepth 1 -type d ! -name '${python3Packages.python.libPrefix}' -print0 | while read -d "" libpkgdir; do
       wrapPythonProgramsIn "$libpkgdir" "$out $pythonPath"
     done
   '';

distros/distro-overlay.nix

@@ -28,12 +28,6 @@ let
     python3 = pythonOverridesFor rosSelf.rosPython or self.python3;
     python3Packages = rosSelf.python3.pkgs;
 
-    # While `python` in Nixpkgs is typically Python 2, this overlay has
-    # historically set it to Python 3 during the ROS transition. Keep it that
-    # way for compatibility.
-    python = rosSelf.python3;
-    pythonPackages = rosSelf.python.pkgs;
-
     boost = self.boost.override {
       python = rosSelf.python;
       enablePython = true;

distros/ros1-overlay.nix

@@ -1,6 +1,12 @@
 self:
 rosSelf: rosSuper: with rosSelf.lib; {
 
+  # While `python` in Nixpkgs is typically Python 2, this overlay has
+  # historically set it to Python 3 as ROS 1 transitioned. Keep it that
+  # way for compatibility.
+  python = rosSelf.python3;
+  pythonPackages = rosSelf.python.pkgs;
+
   # Fix usages of global Boost placeholders
   # https://github.com/ros/actionlib/pull/197
   actionlib = rosSuper.actionlib.overrideAttrs ({