Fix plugin.xml installation path

[Rayman]

Based on testing with the offical pluginlib_export_plugin_description_file macro, it seems that the current way of installing plugin.xml is in the wrong folder. It's one folder too deep. See the difference:

Installation with pluginlib:

pluginlib_export_plugin_description_file(socketcan_interface socketcan_interface_plugin.xml)

find -name '*plugin.xml'
./install/socketcan_interface/share/socketcan_interface/socketcan_interface_plugin.xml

cat ./install/socketcan_interface/share/ament_index/resource_index/socketcan_interface__pluginlib__plugin/socketcan_interface 
share/socketcan_interface/socketcan_interface_plugin.xml

Installed by current code:

find -name socketcan_interface_plugin.xml
./install/socketcan_interface/share/socketcan_interface/socketcan_interface_plugin.xml
./install/socketcan_interface/share/socketcan_interface/socketcan_interface_plugin.xml/socketcan_interface_plugin.xml

cat ./install/socketcan_interface/share/ament_index/resource_index/socketcan_interface__pluginlib__plugin/socketcan_interface 
share/socketcan_interface/socketcan_interface_plugin.xml

After this PR

find -name socketcan_interface_plugin.xml
./install/socketcan_interface/share/socketcan_interface/socketcan_interface_plugin.xml

cat ./install/socketcan_interface/share/ament_index/resource_index/socketcan_interface__pluginlib__plugin/socketcan_interface 
share/socketcan_interface/socketcan_interface_plugin.xml

I still could not verify if the plugin actually loads, but that's for another time.

[mathias-luedtke]

Thanks! It is basically the same fix as #6

[Rayman]

Whoops didn't see that one