The extension manager often uses the extension
relative path to calculate paths to different files,
but does not take into account that this path
already has a trailing slash.

Remove excess slashes to avoid double slashes in paths.

Resolves: #57327
Related: #57312
Releases: 6.2
Change-Id: I60519d4d7075a615d0505bfc4cbb2b6ee1b0c222
Reviewed-on: https://review.typo3.org/28825
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind