Make source_stem required for Rust SourceProviders

This removes ambiguity for developers with the 'stem' property.

Bug: 159064919
Test: New Soong test; error is produced if this property is undefined.
Change-Id: I790fbcfa6cd7dbf6df63b5cf5c22983426ec072e
diff --git a/rust/Android.bp b/rust/Android.bp
index e03bf4f..bbeb28d 100644
--- a/rust/Android.bp
+++ b/rust/Android.bp
@@ -34,6 +34,7 @@
         "library_test.go",
         "project_json_test.go",
         "rust_test.go",
+        "source_provider_test.go",
         "test_test.go",
     ],
     pluginFor: ["soong_build"],