bug#28952: guix pack: Add support for the snap format

[Ricardo Wurmus]

Snaps are app bundles used on Ubuntu.  The bundles is stored in a
squashfs image containing /, /bin, and /meta.

The “meta” directory should contain at least the “snap.yaml” file with
the name, version, synopsis (“summary”), and description of the
package.

For a first version this could already be enough.  In later versions we
could offer ways to customize the contents of the /meta directory.

See this for documentation of the format:

    https://docs.snapcraft.io/snaps/metadata

--
Ricardo

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