Pluginlib ament index resource types should use periods as a separator

[sloretz]

Currently pluginlib_export_plugin_description_file() creates a resource type in the ament index called {plugin category}__pluginlib__plugin. However, This seems like "nesting resource types" to me. The ament resource index documentation says for nested resource types "[...] the convention of using prefixes and suffixes separated by periods should be used.".

For example, RViz plugins are listed as the resource type rviz_common__pluginlib__plugin, but I think resource types like pluginlib.rviz_common or pluginlib.plugin.rviz_common or plugin.rviz_common would be closer to the ament_index documentation.