colcon: enable common extensions by default

With these extensions, colcon behaves as in the default installation
on other distributions.

- colcon-parallel-executor significantly speeds up builds in
  workspaces with multiple packages

- colcon-notification shows build progress summary in terminal title
  and in the last line on the terminal. It also sends desktop
  notification when a build finishes. This can be disabled by setting
  environment variable
  COLCON_EXTENSION_BLOCKLIST=colcon_notification.desktop_notification
  or by the following in ~/.colcon/defaults.yaml:

      build:
        event-handlers:
          - "desktop_notification-"

- colcon-output hides output of successfully built packages. This is
  useful in the case of parallel build and it can be changed via the
  --event-handlers option as in the examples below:

      $ colcon <verb> --event-handlers console_direct+
      $ colcon <verb> --event-handlers console_cohesion+

pkgs/default.nix

@@ -73,8 +73,11 @@ in {
     colcon-devtools
     colcon-library-path
     colcon-metadata
+    colcon-notification
+    colcon-output
     colcon-package-information
     colcon-package-selection
+    colcon-parallel-executor
     colcon-python-setup-py
     colcon-recursive-crawl
     colcon-ros