Except that, as mentioned in TFA, as currently proposed it's possible to say

    #if SOME_PREPROCESSOR_JUNK
    import foo;
    #else
    import bar;
    #endif

This has legitimate use cases, say

    #ifdef WIN32
    import mymodule.windows
    #else
    import mymodule.posix
    #endif

So in reality build systems will be required to invoke at least the preprocessor to extract dependency information.

AFAIK the modules support in the Build2 build system does exactly this, and in fact caches the entire preprocessed file to pass to the compiler proper later.