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 @@ (define %options
         (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 @@ (define (show-help)
 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 @@ (define core-package?
                                 ;; 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