This allows to have an unique identifier for the builder image id against
different depgraphs combinations. The package fingerprint is not enough,
as an atom could have a difference deptree depending on the requires
constraints.
TODO: Don't use the full image name, but only the hash as a salt
(currently the salt contains ALSO a reference of the image-repository,
as such it doesn't allow to port a tree in a different docker registry)
* cmd/search: Add support for search of the packages
with a specific label.
* review Search method of Repositories for permit
different search modes.
* labels are k/v attributes and could be matched
through label key (with HasLabel method) or through
regex that use "$key" + "=" + "$value"
Add "provides" field in packages (which affect both runtime and buildtime deps).
It replaces all the occurences in the deptree before solving, actually
allowing to swap packages and provide virtuals. Along with a mechanism
for package rename #25.
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
