Fix directory based search.

Summary:
canonicalPath() does not include the file name, but only the directory
name. Should use canonicalFilePath() instead.

Test Plan: Manually test with baloosearch

Reviewers: Frameworks, lbeltrame

Reviewed By: lbeltrame

Subscribers: elvisangelaccio, lbeltrame

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5631