guix package: '--show' ignores deprecated packages.

* guix/scripts/package.scm (process-query) <'show>: Remove superseded
packages.
* tests/guix-package-aliases.sh: Add test.

guix/scripts/package.scm

@@ -764,7 +764,8 @@ processed, #f otherwise."
       (('show requested-name)
        (let-values (((name version)
                      (package-name->name+version requested-name)))
-         (match (find-packages-by-name name version)
+         (match (remove package-superseded
+                        (find-packages-by-name name version))
            (()
             (leave (G_ "~a~@[@~a~]: package not found~%") name version))
            (packages

tests/guix-package-aliases.sh

@@ -62,3 +62,6 @@ guix search '\<board\>' game | grep '^name: gnubg'
 guix show --version
 guix show guile
 guix show python@3 | grep "^name: python"
+
+# "python@2" exists but is deprecated; make sure it doesn't show up.
+if guix show python@2; then false; else true; fi