Add test cases for cousin imports that break due to aliases starting with two underscores

betterproto/tests/inputs/import_cousin_package_same_name/cousin.proto

@@ -0,0 +1,6 @@
+syntax = "proto3";
+
+package cousin.subpackage;
+
+message CousinMessage {
+}

betterproto/tests/inputs/import_cousin_package_same_name/test.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+
+package test.subpackage;
+
+import "cousin.proto";
+
+// Verify that we can import a message unrelated to us, in a subpackage with the same name as us.
+
+message Test {
+    cousin.subpackage.CousinMessage message = 1;
+}