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.
This change would actually be a regression for people already using relative names.