A sketch for an adaptors module

[glessard]

A possible design for an adaptors module, to act as an intermediate between types defined in the package (i.e. import SystemPackage) and types defined in the toolchain (i.e. import System).

[glessard]

@swift-ci please test

[FranzBusch]

I was thinking about this a bit and was wondering if you explored just simply doing a @_exported import System in this package on Darwin platforms and guard all the API out? This is similar to what swift-crypto does. AFAIK the APIs of the public SDK and this package are identical on Darwin platforms so this should be a non-API breaking change.