aconfig: improve package identifier test readability

Rearrange the tests in test_is_valid_package_ident to make it more
apparent what constitutes a valid package pattern, and what doesn't.

Bug: 283910447
Test: atest aconfig.test
Change-Id: I3d2b87aed008d0ed3a0aa6e483e655178dda7277
diff --git a/tools/aconfig/src/codegen.rs b/tools/aconfig/src/codegen.rs
index d96d4f9..e50be0b 100644
--- a/tools/aconfig/src/codegen.rs
+++ b/tools/aconfig/src/codegen.rs
@@ -65,8 +65,8 @@
         assert!(is_valid_package_ident("foo_bar_123"));
         assert!(is_valid_package_ident("foo.bar"));
         assert!(is_valid_package_ident("foo.bar.a123"));
-        assert!(!is_valid_package_ident("foo._bar"));
 
+        assert!(!is_valid_package_ident("foo._bar"));
         assert!(!is_valid_package_ident(""));
         assert!(!is_valid_package_ident("123_foo"));
         assert!(!is_valid_package_ident("foo-bar"));