Allow "configurable" classes path in packages
Currently, the class loader expects to find classes inside "Classes" of each package. While this is fine for FLOW3 packages, it blocks (easy) use of other PSR-0 compatible packages - the code always has to be moved into "Classes". To make use of 3rd party code easier, e.g. by using Composer, we need to be able to specify a mapping of package keys to entry points for the class loader.
This change c/should remove the .Shortcuts folder again and could also make "overlapping" package keys possible (e.g. TYPO3.JobQueue and TYPO3.JobQueue.Beanstalk).