A ‘forwarding module’ is a YAML file that’s meant to stand in for a .swiftmodule file and provide an up-to-date description of its dependencies, always using modification times.
When a ‘prebuilt module’ is first loaded, we verify that it’s up-to-date by hashing all of its dependencies. Since this is orders of magnitude slower than reading mtimes, we’ll install a `forwarding module` containing the mtimes of the now-validated dependencies.
We already have something called "module interfaces" -- it's the
generated interface view that you can see in Xcode, the interface
that's meant for developers using a library. Of course, that's also a
textual format. To reduce confusion, rename the new module stability
feature to "parseable [module] interfaces".
...but don't hook it up to anything yet.
This is the very very start of the module stability / textual
interfaces feature described at
https://forums.swift.org/t/plan-for-module-stability/14551/
For now I've just made it a frontend option (not a driver option),
which is good enough for testing.
