Add CompilationSpecs type to handle slices of CompilationSpec, to
perform operation such as Unique() and Remove().
Add also dependencies and the spec associated to the Artifact, to track
how the artifact was generated.
Add revdeps compilation wrapper, and unit tests
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