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/units/miner.rs

@@ -0,0 +1,17 @@
+use crate::app::states::Players;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct MinerUnit {
+    owner: Players,
+    tag: &'static str,
+}
+
+impl MinerUnit {
+    pub fn new(owner: Players) -> Self {
+        Self { owner, tag: "M" }
+    }
+
+    pub fn get_tag(self) -> &'static str {
+        self.tag
+    }
+}