[TASK] ext:install Protect some methods in FolderStructure

Some methods within FolderStructure class hierachy are public
but should be used internally only. The patch protects them.

Change-Id: I273f829f29ca4f5a6da1d03f028be7a8500e222b
Resolves: #52354
Releases: 6.2
Reviewed-on: https://review.typo3.org/24123
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog