DRILL-2815: Augment PathScanner logging; clean up code.
Added some DEBUG- and TRACE-level log calls.
Augmented/edited existing log call.
Misc. code hygiene:
- Added documentation of forResources(...).
- Renamed to clarify a number of local names.
- Made logger private.
- Added "final".
- Fixed indentation; wrapped some long lines; etc.
Conflicts:
common/src/main/java/org/apache/drill/common/util/PathScanner.java
