title: Build Script to write to modulemap file to silence "Missing submodule" warning
body: <p>I am developing a framework, and would like to include my files as submodules. I am currently running a build script to output a universal fat framework that can run on all architectures. However, the framework's module.modulemap file always comes out like so:</p>
  
  <pre><code>framework module MyFramework {
    umbrella header "MyFramework.h"
  
    export *
    module * { export * }
  }
  </code></pre>
  
  <p>When this framework is placed in a sample project, I will get the warning "Missing submodule MyFramework.PublicFile". In order to get this warning to go away, I have to manually input the public files I'd like to include as submodules, so that the module.modulemap file looks like this:</p>
  
  <pre><code>framework module MyFramework {
    umbrella header "MyFramework.h"
    header "PublicFile.h"
    header "OtherPublicFile.h"
    export *
    module * {export *}
  }
  </code></pre>
  
  <p>So the problem is that whenever I rebuild my framework, the module map is reverted to the first file. The other problem I want to avoid having to manually add a lot of public headers to this file as MyFramework grows. Is there any script I can implement either in my Universal Framework script, or in a different build script that I can set once that will write to this file when I compile the framework?</p>