[TASK] Reduce duplicated paths for bowercopy

This makes use of the destPrefix option to set the target base
path only once.

Also drop the leading slash from source path which is unnecessary.

Resolves: #67226
Releases: master
Change-Id: I9d4b19479075a36fd8a7bbab1d65a4e1ce8bcb06
Reviewed-on: http://review.typo3.org/39866


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>