[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:installed }
      "The optional selection hypothesis plugin will not be available since ocamlgraph is not enabled" { !ocamlgraph:installed }
    ]

Reviewed by @samoht
Improve the syntax of filters.

Now is should be similar to the build field, where "%{X}%" can be shortened to X. Also, allow `!`

This is useful for #572
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