rtt_roscomm: Add rosbuild support in rtt_ros service

For being able to use the new rtt_ros service with rosbuild packages, the following updates have been necessary in the rtt_ros import operation:

* Also accept `manifest.xml` as file name for package manifests (using the same `<export><rtt_ros><plugin_depend>` xpath query). 
* Add paths of dependent rosbuild packages (packages that only have a manifest.xml file but no package.xml file) to the search paths list.
* Removed the final import of the package itself as it is already in the `deps_to_import` list.

I tested this version with my rosbuild packages that `plugin_depend` from other rosbuild packages and from installed typekit/library packages from this repository.