[ros2] respect plugin description file subdirectory

[mikaelarguedas]

This modifies the macro to install the plugin description file to match the macro docblock:

pluginlib/pluginlib/cmake/pluginlib_export_plugin_description_file.cmake

Lines 36 to 40 in dfa0036

	# It's relative path to this file will be preserved when being installed.
	# For example, if that argument is "plugins/my_desc.xml", then it will be
	# installed to "<prefix>/share/<package>/plugins/my_desc.xml" or if the
	# argument is just "my_desc.xml" it would be installed to
	# "<prefix>/share/<package>/my_desc.xml".

• Linux
• Linux-aarch64
• macOS
• Windows

@Karsten1987 @wjwwood FYI