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

Update all the imports to import from the crate directly.

board/src/position/position.rs

@@ -2,10 +2,9 @@
 
 use super::{flags::Flags, Pieces};
 use crate::{
-    bitboard::BitBoard,
     moves::Moves,
     piece::{Color, Piece, PiecePlacementError, PlacedPiece, Shape},
-    Square,
+    BitBoard, Square,
 };
 use std::fmt;
 use std::fmt::Write;