A compilespec with an image defined (and no seed) can now be compiled, and an artifact delta is provided at the end of the process.
Reuse the Recipe and extend it to read a separate tree for build dependencies. Also add accessors to compilespec to produce dockerfile image format.
Create compiler interface with switchable backends. For now it can resolve and parse a `build.yaml` which is in the same folder as in `definition.yaml` Add also tests to cover this and new fixtures
