enable find unused items within files

[flickyiyo]

• Added feature to track unused items within used files.
  • When building report now filter used files and pull the ones that have non-empty list inside GraphFile::unused_items.
• Added capability to track items masked through export ... from ... statements.
  • Resolve/Mark items as used inside GraphFile now return an enum if the item was created in the file or it was pulled via export/from statement.