module dhelper is in file 'dhelper.d' which cannot be read

[Milvakili]

I'm new to D and got confused with the dmd.  I assume I have the 
following codes in some folder x with all relevant extra files. 
Should use .di files to import from other modules or the d file 
itself is sufficient to import a module

In c++, gcc -c x/cmain.cpp wil compile without any error.
****cmain.cpp***
#include <iostream>
#include "chelper.h"

int main(){
  std::cout << "Printing from cmain\n" ;
  chelper_foo("cmain");
  return 0;
}

In D, dmd -c x/dmain.d will complainsabout dhelper.d file which 
is in the x folder.  What should I do to get  rid of this error?  
dmd -c x/dmain.d x/dhelper.d compiles but I do not want that.

****dmain.d***
import std.stdio;
import dhelper;

void main(){
  writefln("Hi from d");
  dhelper_foo("dmain");
}