Bazel client: explain the name of A-server.jar

Also add a helper method to GlobalVariables to
retrieve this path, thus concentrating the
assumptions about the layout of extracted_binaries
in one place.

Change-Id: If172b6f5bf4451845ad89d3d488ef2a2c2e5d286
PiperOrigin-RevId: 158241854
diff --git a/src/main/cpp/global_variables.h b/src/main/cpp/global_variables.h
index 4774f29..061e283 100644
--- a/src/main/cpp/global_variables.h
+++ b/src/main/cpp/global_variables.h
@@ -44,6 +44,12 @@
 struct GlobalVariables {
   GlobalVariables(OptionProcessor *option_processor);
 
+  std::string ServerJarPath() const {
+    // The server jar is called "A-server.jar" so it's the first binary we
+    // extracted.
+    return extracted_binaries.empty() ? "" : extracted_binaries[0];
+  }
+
   // Used to make concurrent invocations of this program safe.
   std::string lockfile;  // = <output_base>/lock