[BUGFIX] Class loader tries to load classes it cannot load

If a class file from a third-party package is loaded which does not
follow the Vendor\Product\Classname convention (e.g. Product\Classname),
the class loader still tries to access the third component of the class
name, leading to a warning (and thus an exception, depending on your
configuration).

To fix this problem, the class loader now checks in advance if it can
possibly load this class (or rather if the classname conforms to the
specification for the CMS universe).

Change-Id: I1cd3964b9486ea591fb373533fd6cef4e9ce3989
Resolves: #50217
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22629
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind