[BUGFIX] Fix relative path calculations in EM

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