server/buildfile.go -> pkg/buildfile/buildfile.go

Moves the buildfile out of the doomed `server/` directory into its own package.

Portability notes:

NewBuildFile now takes instances of daemon and engine, because passing the server was causing a circular dependency. FWIW, it doesn't need the server object itself.

server.ImageGetCached did not need a server object either, but was the only additional dependency in the buildfile for a server object. Moved to Daemon and now accepts a daemon object instead of the server object.

All tests pass. Please let me know if there are any concerns.