find_and_undo: no need to kill_thy again -- Thy_Info.toplevel_begin_theory does that initially (cf. 3ceccd415145);

renamed funny Library ROOT files back to default ROOT.ML -- ML files are no longer located via implicit load path (cf. 2b9bfa0b44f1);

tuned headers -- more precise load path;

theory loading: only the master source file is looked-up in the implicit load path;

load_thy: refer to physical master directory (not accumulated source import directory) and enable loading files relatively to that;

simplified/clarified Thy_Load path: search for master only, lookup other files relative to that; tuned;