DRILL-5533: Fix flag assignment in FunctionInitializer.checkInit() method
Changes:
1. Fixed DCL in FunctionInitializer.checkInit() method (update flag parameter when function body is loaded).
2. Fixed ImportGrabber.getImports() method to return the list with imports.
3. Added unit tests for FunctionInitializer.
4. Minor refactoring (renamed methods, added javadoc).
closes #843
