use the -impl, not .impl suffix for autogenerated stubs

b/31055568

Change-Id: I4fc4dca9521a0d2149ac33ca40e4d7b473e492f7
diff --git a/main.cpp b/main.cpp
index 9dae1c2..134cc6b 100644
--- a/main.cpp
+++ b/main.cpp
@@ -418,7 +418,7 @@
         Coordinator *coordinator,
         const std::string &outputDir) {
 
-    const std::string libraryName = makeLibraryName(packageFQName) + ".impl";
+    const std::string libraryName = makeLibraryName(packageFQName) + "-impl";
 
     std::vector<FQName> packageInterfaces;