title: Why aren't other modules being compiled?
body: <p>I have two files: <code>Main.d</code> and <code>ImportMe.d</code>.  Their purposes should be self-explanatory.  They are in the same directory, and have no explicit module declaration.  When I try to compile <code>Main.d</code>, though, I get a "symbols not found" error!</p>
  
  <pre><code>$ dmd Main.d -I.
  
  Undefined symbols:
    "_D8ImportMe12__ModuleInfoZ", referenced from:
        _D4Main12__ModuleInfoZ in Main.o
    "_D8ImportMe8SayHelloFxAyaZv", referenced from:
        __Dmain in Main.o
  ld: symbol(s) not found
  collect2: ld returned 1 exit status
  --- errorlevel 1
  </code></pre>
  
  <p>Compiling both files at the same time works fine.</p>
  
  <pre><code>$ dmd Main.d ImportMe.d
  </code></pre>
  
  <p>You don't have to do this with the standard library, though.  What is it doing differently?  Changing the include path via <code>-I</code> has no visible effect.</p>