JENKINS-25011: find projects by their full name.

https://wiki.jenkins-ci.org/display/JENKINS/Hierarchical+projects+support

Using `Jenkins.getInstance().getItem(...)` only returns `TopLevelItem`, which
means that it was previously impossible to use the `Promoted Build Parameter`
with folders. To make that work better, use full names, and let autocomplete
return full names.
Right, saw that later. Given the difficult finding the context when it is needed, perhaps it is this one which should be open.