druntime homebrew: setup and compile a single project with its own stdlib

[Dmitry Ponyatov]

> If you have a module source file with the same name you want to 
> replace, the build will prioritize your own version.

You mean I can create `project/druntime`, and replace the core D 
runtime part by part by placing here my own files from druntime 
source?

And compiler will route his internal callbacks etc into my own 
partial stdlib without any undocumented options or .a/.so 
prebuilding ?