[board] Clean up a bunch of build errors

Fix imports to refer to core and bitboard crates.
Fix some API use errors.

board/src/position/position.rs

@@ -7,8 +7,9 @@ use crate::{
     position::DiagramFormatter,
     r#move::Castle,
     sight::Sight,
-    BitBoard, Move,
+    Move,
 };
+use chessfriend_bitboard::BitBoard;
 use chessfriend_core::Square;
 use std::{cell::OnceCell, fmt};