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.
More readable if these methods are just inlined, so your eyes do not have to scan back and forth.