[FEATURE] Allow "configurable" classes path in packages
Package instances can be asked for their classes path, which is now set
determined by the PackageManager and stored in PackageStates.php.
It can - in the future - thus be read from a Composer file and
does no longer have to be the hardcoded "Classes".
This change removes the use of the .Shortcuts folder again and makes
"overlapping" package keys possible (e.g. TYPO3.JobQueue and