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

Update all the imports to import from the crate directly.

board/src/moves/king.rs

@@ -5,10 +5,9 @@
 
 use super::{move_generator_declaration, MoveGeneratorInternal, MoveSet};
 use crate::{
-    bitboard::BitBoard,
     piece::{Color, Piece, PlacedPiece},
     position::BoardSide,
-    Move, Position,
+    BitBoard, Move, Position,
 };
 
 move_generator_declaration!(KingMoveGenerator, struct);