[BUGFIX] Extension upload as zip on Windows systems ignores directories

Using DIRECTORY_SEPARATOR on Windows environments is a "\" instead of
a "/". Fix this by replacing the constant with '/'.

Resolves: #53740
Resolves: #53346
Releases: 6.2
Change-Id: Ia745a17fd47c527b57e78babef76b67f30af0454
Reviewed-on: https://review.typo3.org/26911
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes