Add ros2plugin (backport #165)

[mergify]

Add the plugin command together with the list verb to ros2 CLI.

At the moment it produces the following results:

$ ros2 plugin list
demo_plugin_component:
	Plugin(name='ros2_playground::MessageWriterDerived', type='ros2_playground::MessageWriterDerived', base='ros2_playground::MessageWriter')
rviz_default_plugins:
	Plugin(name='rviz_default_plugins/Camera', type='rviz_default_plugins::displays::CameraDisplay', base='rviz_common::Display')
	Plugin(name='rviz_default_plugins/Grid', type='rviz_default_plugins::displays::GridDisplay', base='rviz_common::Display')
	...

while passing the flag --packages only list packages that export plugins and the relative path to the XML file(s ?),

$ ros2 plugin list --packages
demo_plugin_component:
	share/demo_plugin_component/plugins_description.xml
rviz_default_plugins:
	share/rviz_default_plugins/plugins_description.xml

I'd be more than open to discuss actual output and/or output layout.

Note this does not exactly mimic ROS 1 in terms of prints layout nor in terms of use (yet),
e.g. ROS 1 --> rospack plugins --attrib=plugin <base_package_name>.

Fix ros2/ros2cli#328
Replace ros2/ros2cli#340

Signed-off-by: artivis [email protected]

---

This is an automatic backport of pull request #165 done by Mergify.

[mergify]

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Changes to be committed:
	new file:   ros2plugin/package.xml
	new file:   ros2plugin/resource/ros2plugin
	new file:   ros2plugin/ros2plugin/__init__.py
	new file:   ros2plugin/ros2plugin/api/__init__.py
	new file:   ros2plugin/ros2plugin/command/__init__.py
	new file:   ros2plugin/ros2plugin/command/plugin.py
	new file:   ros2plugin/ros2plugin/verb/__init__.py
	new file:   ros2plugin/ros2plugin/verb/list.py
	new file:   ros2plugin/setup.py
	new file:   ros2plugin/test/test_copyright.py
	new file:   ros2plugin/test/test_flake8.py
	new file:   ros2plugin/test/test_pep257.py
	new file:   ros2plugin/test/test_xmllint.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both added:      pluginlib/include/pluginlib/class_desc.hpp
	both added:      pluginlib/include/pluginlib/class_list_macros.hpp
	both added:      pluginlib/include/pluginlib/impl/split.hpp
	deleted by us:   pluginlib/src/list_plugins.cpp
	deleted by us:   pluginlib/test/include/test_base.hpp
	deleted by us:   pluginlib/test/include/test_plugins.hpp
	both added:      pluginlib/test/include/visibility_control.hpp
	both added:      pluginlib/test/test_plugins.cpp

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