Support importing synthetic declarations from "magic" forward
declaration packages:
- cnames.structs
- objcnames.classes
- objcnames.protocols
So this is a rough equivalent to the forward declarations module
made of ForwardDeclarationsPackageFragmentDescriptor in K1.
Unlike K1 implementation, this K2 one doesn't allow importing
a declaration that wasn't actually forward-declared in a C/Objective-C
header available through a cinterop klib dependency.
Svyatoslav Scherbina
b19058caef