Refactor(Structure): Move ADTs into adt/ folder, rename spel.rkt to main.rkt

New structure groups all ADT modules under adt/ directory, removing
redundant adt- prefix from filenames. Library names now read as
(pacman-project adt position) etc. All imports updated accordingly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pacman-project/tests/test-position.rkt

@@ -7,7 +7,7 @@
 (define-library (pacman-project tests test-position)
   (import (scheme base)
           (pp1 tests)
-          (pacman-project adt-position))
+          (pacman-project adt position))
   (export test)
 
   (begin