...instead of platforms.
Notable changes/flags:
* Append to variables controlling paths and names, to allow for user
configuration
* add `SwiftCore_USE_STATIC_LIBS` to generate static archives
* use PlatformInfo variables to get the platform and arch subfolders
(where appropriate)
* add include guards to ensure PlatformInfo and FindSwiftCore are
included once in a project
* search for the appropriate static or import library under Windows
Addresses rdar://152838903
Adds common CMAKE modules to be used by the supplemental libraries.
Namely:
* ResourceEmbedding
* PlatformInfo
* gyb
* EmitSwiftInterface
* CatalystSupport
