Issue #2083547 by donquixote: Docblock and code style improvements in DrupalKernel.

This is in preparation for the switch of module-provided classes from PSR-0 to PSR-4.
Issue #2083547 by donquixote: Implement PSR-4 for module-provided class files.

This affects class loading, plugin discovery and test discovery.

PSR-0 is still supported, and no module class files are moved yet.
Issue #2083547 by donquixote: Introduce core/scripts/switch-psr4.sh, to move module-provided class files to their new PSR-4 location.
Issue #2083547 by donquixote: Move class files in core/modules/shortcut to their new PSR-4 location, to verify that PSR-4 actually works.