Use ServiceGenerator logging conventions some as other tools.

Some other tooling is using NAME.VERSION.json for the file names, so follow that
style.

They also use "index.json" for the directory listing, so use that also.

Tools/ServiceGenerator/SGMain.m

@@ -435,7 +435,7 @@ - (void)printUsage:(FILE *)outputFile brief:(BOOL)brief {
 - (void)maybeLogAPI:(GTLRDiscovery_RestDescription *)api {
   if (self.apiLogDir == nil) return;
 
-  NSString *fileName = [NSString stringWithFormat:@"%@_%@.json",
+  NSString *fileName = [NSString stringWithFormat:@"%@.%@.json",
                         api.name, api.version];
   NSString *filePath = [self.apiLogDir stringByAppendingPathComponent:fileName];
   NSError *writeErr = nil;
@@ -1186,7 +1186,7 @@ - (void)stateDirectory {
       } else {
         GTLRDiscovery_DirectoryList *apiList = (GTLRDiscovery_DirectoryList *)object;
         if (self.apiLogDir != nil) {
-          NSString *filePath = [self.apiLogDir stringByAppendingPathComponent:@"DirectoryList.json"];
+          NSString *filePath = [self.apiLogDir stringByAppendingPathComponent:@"index.json"];
           NSError *writeErr = nil;
           if (![apiList.JSONString writeToFile:filePath
                                     atomically:YES