Fixes #1384. Fixes Scope.parseScopedKey

Scope.parseScopedKey now supports full range of legal keys
described in the documentation including {.} and other
notation for ProjectRef, BuildRef, and ThisBuild.
Deprecate parseScopedKey
Merge pull request #1493 from sbt/fix/1384

Fixes #1384. Fixes Scope.parseScopedKey