[KCoreDirLister] replace deprecated foreach with range-for

Summary: Also change directoriesForCanonicalPath() to return const

Test Plan:
The code compiles and all tests pass (except kacltest, but that's
probably something on my machine)

Reviewers: kde-frameworks-devel, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23902