fix exported plugin xml for collision_distance_field

[v4hn]

Otherwise the xml can not be found on an installed workspace

[v4hn]

should be picked to j/k

[davetcoleman]

I don't understand why having the install command in a subdirectory causes a problem. Don't the CMakeLists behave the same when you use add_subdirectory(collision_distance_field) ?

[v4hn]

The problem is not the install rule in the subfolder, but the install location of the xml file. It should have the same relative path in the source folder of the package and in the installed share-folder, because only one relative path can be specified in the `package.xml` and the file has to be available in devel and install workspaces. I didn't want to create `<install-ws>/share/moveit_experimental/collision_distance_field`, so instead I moved the file one level higher in the packages source folder.

[davetcoleman]

picked to j/k