Support nested messages, fix casing. Support test-cases in packages.

betterproto/tests/inputs/import_root_package_from_child/child.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+
+package child;
+
+import "root.proto";
+
+// Verify that we can import root message from child package
+
+message Test {
+    RootMessage message = 1;
+}

betterproto/tests/inputs/import_root_package_from_child/import_root_package_from_child.proto

@@ -1,11 +0,0 @@
-syntax = "proto3";
-
-import "root.proto";
-
-package child;
-
-// Tests generated imports when a message inside a child-package refers to a message defined in the root.
-
-message Test {
-  RootMessage message = 1;
-}