[BUGFIX] Load XML files of Extension Manager properly

Since the XEE security fix (I26701fc2ffb5aed7ccbd96c168aef571d012091e),
the XML files in the Extension Manager may are not loaded anymore, depending
on the machine. Change the way how the files are loaded to fix the issue.

Change-Id: I2a3dffd089ed427b965bcbae8aa596c26a81770b
Resolves: #75022
Releases: master, 7.6, 6.2
Reviewed-on: https://review.typo3.org/47196


Reviewed-by: Daniel Maier <dani-maier@gmx.de>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>