flake.nix: Expose all top-level packages

Previously, the legacyPackages output contained only the content of
the rosPackages attribute, but things like colcon, superflore or
gazebo were not available there.

With this change, we expose all packages defined in pkgs/default.nix
as well as the rosPackages attribute. This is the same as when using
just default.nix without flakes. This means that one can simply use

    nix shell .#colcon

to have colcon command available.

To maintain backward compatibility, we still keep ROS distro package
sets at the top level. Therefore, the same ROS package is available at
two places. For example, ros-core from humble is available as:

- .#humble.ros-core (before and after this commit)
- .#rosPackages.humble.ros-core (after this commit)

flake.nix

@@ -15,7 +15,8 @@
         overlays = [ self.overlays.default ];
       };
     in {
-      legacyPackages = pkgs.rosPackages;
+      legacyPackages = (intersectAttrs (self.overlays.default null pkgs) pkgs)
+                       // pkgs.rosPackages;
 
       devShells = {
         example-turtlebot3-gazebo = import ./examples/turtlebot3-gazebo.nix { inherit pkgs; };