change the interface to get a project item for a url. This implementation is a lot slower than the old one, however the old one was incorrect in two ways:

- it assumed there's only 1 project item for a given url, thats wrong as a file might be part of multiple targets
- it assumed that the project item for a given url is inside the subtree that has the same path, which might not be the case

In the cause of this, the function now returns a list of items, that is all these items have the same url, but different parents. The language support parts probably want to do something more sensible than just taking the first one.