swift-package-manager

[Build] Link XCTest with Foundation

In order to implement asynchronous tests, swift-corelibs-xctest
will take on a dependency upon swift-corelibs-foundation. To
properly link the two:

- Allow the package manager to take a path to a Foundation build.
- Add the Foundation and CoreFoundation directories to the swiftc
  include paths.
- While we're at it, remove the SWIFTPM_EXTRA_IMPORTS hack in
  favor of -Xswiftc.