[board] Export BitBoard directly off of the crate for internal use

Update all the imports to import from the crate directly.

board/src/moves/pawn.rs

@@ -1,9 +1,8 @@
 // Eryn Wells <eryn@erynwells.me>
 
 use crate::{
-    bitboard::BitBoard,
     piece::{Color, Piece, Shape},
-    Move, Position,
+    BitBoard, Move, Position,
 };
 
 enum MoveList {