JENKINS-25011: find projects by their relative names

[JENKINS-25011](https://issues.jenkins-ci.org/browse/JENKINS-25011)

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 relative names.

This is an alternative to pull request #47 
This code was fine as it stood. `AbstractItem.getRelativeNameFrom(Item)` delegates to `getRelativeNameFrom(ItemGroup)`.