profiles: 'package->manifest-entry' preserves transformations by default.

profiles: 'package->manifest-entry' preserves transformations by default.

Previously, transformations applied from a manifest (rather than via
"guix install") would be lost.  This change fixes that and simplifies
things.

Reported by zimoun at
<https://lists.gnu.org/archive/html/guix-devel/2021-02/msg00153.html>.

* guix/profiles.scm (default-properties): New procedure.
(package->manifest-entry): Use it for #:properties.
* guix/scripts/pack.scm (guix-pack)[with-transformations]: Remove.
Remove caller.
* guix/scripts/package.scm (transaction-upgrade-entry): Remove calls to
'manifest-entry-with-transformations'.
* tests/guix-package.sh: Add test.
* tests/transformations.scm ("options->transformation + package->manifest-entry"):
New test.