core: add per-package and per-filesystem show-info

core: add per-package and per-filesystem show-info

Sometimes, it is need to quickly get the metadata of a subset of
packages, without resorting to a full-blown JSON query.

Introduce a new per-package (and per-filesystem) foo-show-info rule,
that otputs a per-entity valid JSON blob.

Note that calling it for multiple packages and.or filesystems at once
will not generate a valid JSON blob, as there would be no separator
between the JSON elements:

    $ make {foo,bar}-show-info
    { "foo": { foo stuff } }
    { "bar": { bar stuff } }

However, jq is able to absorb this, with its slurping ability, which
generates an array (ellipsed and manualy reformated for readability):

    $ make {foo,bar}-show-info |jq -s . -
    [
      { "foo": { foo stuff } },
      { "bar": { bar stuff } }
    ]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>