Declare module at top of file rather than bottom.

This is the usual style.

Test: mm
Change-Id: I95602b2c0e310b80aedd8f4f89baa99e50331c4f
diff --git a/microdroid_manager/src/payload.rs b/microdroid_manager/src/payload.rs
index 8ec6f74..b731d33 100644
--- a/microdroid_manager/src/payload.rs
+++ b/microdroid_manager/src/payload.rs
@@ -14,6 +14,8 @@
 
 //! Routines for handling payload
 
+mod apex;
+
 use crate::instance::ApexData;
 use crate::ioutil::wait_for_file;
 use anyhow::Result;
@@ -61,5 +63,3 @@
         ..Default::default()
     }
 }
-
-mod apex;