[TASK] Include PSR/Log as composer package

The interface for the PSR standard "PSR/log" (PSR-3)
is now migrated to a composer package which resides
inside typo3/contrib/vendor and loads all files of that
package.

Resolves: #65954
Releases: master
Change-Id: I423a41b6116693903d7d374e4f252d2932297a5f
Reviewed-on: http://review.typo3.org/38066


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>