Remove dependency on SourceKitten (#29)

* Remove dependency on SourceKitten

* Fix missing imports

Link to the static version of SwiftToolsSupport

Update to new SHA256() API in the core (#27)

* Update to new SHA256() API in the core
* Move to Xcode 11.4 to get MacOS 10.15 (which should have CryptoKit)

Move to using the new swift-tools-support-core (#26)

- This new framework has the useful bits pulled out of the SPM project

Swift Package Manager compatible version tag.

Add a path argument to log messages (#21)

- When specified the log messages match those typically output by compilers
- This means that Xcode will properly parse them and you'll see them in the issues
  pane on the left, and clicking them will take you to the right file, etc.