[FEATURE] Check extension key before upload

Fixes the problem with uploading extensions with capital
chars in the extension key

Change-Id: I592deaa75d03fdb0dd5aa820c04541f3a5fae57d
Resolves: #57759
Reviewed-on: http://review.typo3.org/29844
Reviewed-by: Christian Zenker <typo3@xopn.de>
Tested-by: Christian Zenker <typo3@xopn.de>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Thomas Löffler <loeffler@spooner-web.de>
Tested-by: Thomas Löffler <loeffler@spooner-web.de>