Update ES6 imports with comment about corresponding proto import path.

I was experiencing a long import path in the generated code that doesn't
work. Outputting the proto file that corresponds to a JS import is helpful for
both diagnosing the issue and writing a wrapper for altering the import path to
my satisfaction.

Before:

```js
import * as github_com_gonzojive_rules_ts_proto_example_prefix_greeting_pb from '../../../../../github.com/gonzojive/rules_ts_proto/example/prefix/greeting_pb';
```

After:

```js
import * as github_com_gonzojive_rules_ts_proto_example_prefix_greeting_pb from '../../../../../github.com/gonzojive/rules_ts_proto/example/prefix/greeting_pb'; // proto import: "github.com/gonzojive/rules_ts_proto/example/prefix/greeting.proto"
```

javascript/net/grpc/web/generator/grpc_generator.cc

@@ -555,15 +555,18 @@ void PrintES6Imports(Printer* printer, const FileDescriptor* file) {
 
   std::set<string> imports;
   for (const auto& entry : GetAllMessages(file)) {
-    const string& name = entry.second->file()->name();
+    const string& proto_filename = entry.second->file()->name();
     string dep_filename = GetRootPath(file->name(), name) + StripProto(name);
     if (imports.find(dep_filename) != imports.end()) {
       continue;
     }
     imports.insert(dep_filename);
     // We need to give each cross-file import an alias.
-    printer->Print("import * as $alias$ from '$dep_filename$_pb';\n", "alias",
-                   ModuleAlias(name), "dep_filename", dep_filename);
+    printer->Print("import * as $alias$ from '$dep_filename$_pb'; // proto import: \"$proto_filename$\"\n",
+                   "alias", ModuleAlias(name),
+                   "dep_filename", dep_filename,
+                   "proto_filename", proto_filename);
+  }
   }
   printer->Print("\n\n");
 }