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

betterproto/tests/inputs/import_cousin_package/cousin.proto

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

betterproto/tests/inputs/import_cousin_package/test.proto

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