packages: Add 'package-superseded' and associated support.

This provides a way to mark a package as superseded by another one.
Upgrades replace superseded packages with their replacement.

* guix/packages.scm (package-superseded, deprecated-package): New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* gnu/packages.scm (%find-package): Check for 'package-superseded'.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  Call it when 'package-superseded' is true.
* tests/guix-build.sh: Add test for a superseded package.
* tests/packages.scm ("package-superseded")
("transaction-upgrade-entry, superseded package"): New tests.

1 files changed, 8 insertions, 1 deletions

diff --git a/gnu/packages.scm b/gnu/packages.scm
index 68a9eef2ad9..5d60423a3ae 100644
--- a/gnu/packages.scm
+++ b/gnu/packages.scm
@@ -305,7 +305,14 @@ return its return value."
      (when fallback?
        (warning (_ "deprecated NAME-VERSION syntax; \
 use NAME@VERSION instead~%")))
-     pkg)
+
+     (match (package-superseded pkg)
+       ((? package? new)
+        (info (_ "package '~a' has been superseded by '~a'~%")
+              (package-name pkg) (package-name new))
+        new)
+       (#f
+        pkg)))
     (_
      (if version
          (leave (_ "~A: package not found for version ~a~%") name version)