Revert "Revert package separation for now"

This reverts commit 89749c3aa7f94546ea692207998043bcd0d5f792.
Merge branch 'develop' of github.com:fsharp/FAKE into develop
Merge branch 'develop' of github.com:fsharp/FAKE into develop
Package separation create a Core package. We don't want to break the FAKE pacakage for now