packages: Add 'package-superseded' and associated support.

packages: Add 'package-superseded' and associated support.

This provides a way to mark a package as superseded by another one.
Upgrades replace superseded packages with their replacement.

* guix/packages.scm (package-superseded, deprecated-package): New
procedures.
* gnu/packages.scm (%find-package): Check for 'package-superseded'.
* guix/scripts/package.scm (transaction-upgrade-entry)[supersede]: New
procedure.  Call it when 'package-superseded' is true.
* tests/guix-build.sh: Add test for a superseded package.
* tests/packages.scm ("package-superseded")
("transaction-upgrade-entry, superseded package"): New tests.