Issue 11666: Separate each platforms ports into its own directory (via package.d)

Likewise to #731, this is not suitable for merging yet.  This doesn't implement splitting architecture specific differences like my proposal in [issue 11666](https://d.puremagic.com/issues/show_bug.cgi?id=11666), but instead leverages a recent D feature to achieve similar, and hopefully just as manageable separation of modules to ease the code complexity of adding new ports.

This pull instead tackles the problem by moving ``config.d`` into a new ``config/package.d`` package, and any arch-specific definitions be created as sub-modules of this package.  This pull starts off with ``core.sys.posix``, but it's intention is that other places that have (or could have) a config module get turned into package too.  eg:  ``core.sys.linux``, ``core.sys.freebsd``, ``core.sys.solaris``, ``core.sys.osx``, ``core.sys.windows``, ``core.stdc``, etc...