refresh: Add '--expression'.

* guix/scripts/refresh.scm (%options, show-help): Add --expression.
(guix-refresh): Honor it.
* doc/guix.texi (Invoking guix refresh): Document it.

doc/guix.texi

@@ -4277,6 +4277,19 @@ The following options are supported:
 
 @table @code
 
+@item --expression=@var{expr}
+@itemx -e @var{expr}
+Consider the package @var{expr} evaluates to.
+
+This is useful to precisely refer to a package, as in this example:
+
+@example
+guix refresh -l -e '(@@@@ (gnu packages commencement) glibc-final)'
+@end example
+
+This command lists the dependents of the ``final'' libc (essentially all
+the packages.)
+
 @item --update
 @itemx -u
 Update distribution source files (package recipes) in place.  This is

guix/scripts/refresh.scm

@@ -80,6 +80,9 @@
         (option '(#\L "list-updaters") #f #f
                 (lambda args
                   (list-updaters-and-exit)))
+        (option '(#\e "expression") #t #f
+                (lambda (opt name arg result)
+                  (alist-cons 'expression arg result)))
         (option '(#\l "list-dependent") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'list-dependent? #t result)))
@@ -115,6 +118,8 @@ Update package definitions to match the latest upstream version.
 When PACKAGE... is given, update only the specified packages.  Otherwise
 update all the packages of the distribution, or the subset thereof
 specified with `--select'.\n"))
+  (display (_ "
+  -e, --expression=EXPR  consider the package EXPR evaluates to"))
   (display (_ "
   -u, --update           update source files in place"))
   (display (_ "
@@ -348,6 +353,8 @@ update would trigger a complete rebuild."
                                 ;; Take either the specified version or the
                                 ;; latest one.
                                 (specification->package spec))
+                               (('expression . exp)
+                                (read/eval-package-expression exp))
                                (_ #f))
                              opts)
             (()                                   ; default to all packages