[Client] packages can specify messages to print during summary

Add a messages field that allows to describe which feature is missing
when an optional dependency is not installed (issue #572)

For instance:

    messages: [
      "The optional realizations and why3's coq plugin will not be available since coq is not enabled" {"%{coq:enable}%" = "disable"}
      "The optional selection hypothesis plugin will not be available since ocamlgraph is not enabled" {"%{ocamlgraph:enable}%" = "disable"}
    ]