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 
The minimum would be to retain the path of the `context` as a serialized field, though this is also undesirable. The problem is how to get at the context from `getBuilds`, used by `getDefaultParameterValue`. I am unsure whether this is possible without taking the approach of #47 and forcing full names.