Debugger: Implement #10970.

- If a source file cannot be found, and the user clicks the item to locate it,
  Debugger now attempts to query all volumes for the corresponding filename.
  If any are found, they are presented in a menu, along with an option to
  locate the file manually as before (since the file may potentially live on
  an unindexed volume). If no matches are found via query, we fall through to
  the file panel directly as before.