cache-lookahead refactoring - part 2 (#518)

* Revert "Cache lookahead refactoring (#330)"

This reverts commit 4a6139a.

* pass opts via callgraph

* convert to functions where possible

* pass cache key via callgraph

* pass filecontext via callgraph

* move optimize out of build

* pass layerCache via callgraph

* turn pushLayerToCache into a global mock

* pass snapshotter through callgraph

* pass crossStageDeps via callgraph

* pass image via callgraph

* cleanup

* typo

* cleanup

* Reapply "Cache lookahead refactoring (#330)"

This reverts commit d7b6d58.

* cleanup

* fixup

* cleanup

* cleanup

* cleanup

Author: mzihlmann