Added automatic backup for debugging incremental compilation.
Fix: not loading ancestor packages from incremental cache.
Fixed opening file and not closing it.

This led to impossibility to delete file during incremental compilation.
Fixed clearing incremental cache for removed source files.

For writing system-dependent paths were used, for deleting were system-independent. That's why cache was not cleared on Windows.
Minor. Clarified "compiled file" as "file to compile".