Move buildings and units under skirmish_states

Reorganize the codebase by moving the buildings and units modules
into the skirmish_states namespace, updating imports accordingly.

src/app/states/skirmish_states/mod.rs

@@ -1,6 +1,8 @@
 mod board;
+pub mod buildings;
 mod focused_cell;
 mod offset;
+pub mod units;
 
 pub use board::BoardState;
 pub use focused_cell::{FocusedCell, MoveFocusedCell};