R1:95c9d403f4a3

Make objects implementing BuildableInterface produce a BuildableEngine

Summary:
Ref T13110. Currently, build status is published the same way for every Buildable by the BuildEngine.

I want to change this to delegate publishing to each Buildable, particularly so that Differential may use more detailed rules for handling builds and drafts.

Rather than add additional methods to the existing `BuildableInterface`, add an engine generator method instead. This is a pattern which has seen more use recently (e.g.