Modules seem to be sidestepped. I wonder why is that, they would make the languages so much easier to work with. One reason is that they might involve a big overhaul of toolchains, I think...
It was submitted for PDTS (proposed draft technical standard), which means it goes out for balloting to ISO national bodies. That balloting either succeeds (at which point you have a Techincal Specification) or it comes back to the committee with comments that the committee then has to respond to (and then another round of balloting happens). Once it's accepted as a Technical Specification, it's still not part of the C++ International Standard. That's a decision the committee comes to at their own pace, much like what happened at this meeting where the Concepts TS was accepted (as amended) into the IS.
Think of a TS somewhat like a beta. There's no assurance that the contents of the TS will go into the IS at all or in the exact form it was originally specified in, but that's certainly the hope when the TS is published.
And on the ground things are looking pretty good: the big three (GCC, Clang, and VC) have modules supported to various degrees and are actively working on improving things.
There are a couple nice implementations already, so I'd expect no problem with a modules proposal making the cut for the next standard.
A simple solution is already possible with Microsoft's implementation, but they don't like it, it seems.
#ifndef LEGACY_HEADER
#define LEGACY_HEADER
#include "legacy_header_macros.h"
import new_header_module;
#endif