Error parsing xml comment in package.xml

[ashwinvkNV-zz]

Issue:
When a comment(eg: license) is added using the <!-- example_comment --> format before the tag in the package.xml file of a rviz plugin, pluginlib exits with the following error:
Could not find a root element for package manifest at
This is because the tinyxml2::XMLDocument function FirstChildElement("package") returns NULL when a XML comment is present before the "package" child element. https://github.com/ros/pluginlib/blob/ros2/pluginlib/include/pluginlib/class_loader_imp.hpp#L300

Branch: ros2

Commit Hash: b0388a8

Workaround:
Adding the comment using a non-existing processing-instruction works as expected.

eg:

<?comment
  example comment
?>