core: introduce new global show-info

core: introduce new global show-info

Users are increasingly trying to extract information about packages. For
example, they might need to get the list of URIs, or the dependencies of
a package.

Although we do have a bunch of rules to generate some of that, this is
done in ad-hoc way, with most of the output formats just ad-hoc, raw,
unformatted blurbs, mostly internal data dumped as-is.

Introduce a new rule, show-info, that provides a properly formatted
output of all the meta-information about packages: name, type, version,
licenses, dependencies...

We choose to use JSON as the output format, because it is pretty
versatile, has parsers in virtually all languages, has tools to parse
from the shell (jq). It also closely matches Python data structure,
which makes it easy to use with our own internal tools as well. Finally,
JSON being a key-value store, allows for easy expanding the output
without requiring existing consumers to be updated; new, unknown keys
are simply ignored by those (as long as they are true JSON parsers).

The complex part of this change was the conditional output of parts of
the data: virtual packages have no source, version, license or
downloads, unlike non-virtual packages. Same goes for filesystems. We
use a wrapper macro, show-info, that de-multiplexes unto either the
package-related- or filesystem-related macros, and for packages, we also
use a detailed macro for non-virtual packages.

It is non-trivial to properly output correct JSON blurbs, especially
when trying to output an array of objects, like so, where the last item
shall not be followed by a comma:  [ { ... }, { ... } ]

So, we use a trick (as sugegsted by Arnout), to $(subst) any pair of
",}" or ", }" or ",]" or ", ]" with only the respective closing symbol,
"}" or "]".

The whole stuff is $(strip)ed to make it a somewhat-minified JSON blurb
that fits on a single line with all spaces squashed (but still with
spaces, as it is not possible to differentiate spaces between JSON
elements from spaces inside JSON strings).

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