[BUGFIX] False positive check for read-only extension

Extensions which are not loaded are not reliably checked for
write access to activate the OOo->reST online converter tool.

Change-Id: I4ca55a6033fb351e3ea8d496da6bf55b276526b2
Fixes: #58419
Reviewed-on: https://review.typo3.org/29788
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers