share code between file scanners

Summary: move most of the scanning of files to the helper file so that it can be shared and allow subsequent improvements to both code paths

Test Plan: indexing still works correctly

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D12718