weather: Accept package specs on the command line.

Previously, non-option arguments would be ignored.  Now it puts them to
good use.

* guix/scripts/weather.scm (guix-weather)[package-list]: New procedure.
Use it.
* doc/guix.texi (Invoking guix weather): Adjust accordingly.

doc/guix.texi

@@ -10089,8 +10089,17 @@ challenge}, it ignores signatures on those substitutes, which is
 innocuous since the command only gathers statistics and cannot install
 those substitutes.
 
-Among other things, it is possible to query specific system types and
-specific package sets.  The available options are listed below.
+The general syntax is:
+
+@example
+guix weather @var{options}@dots{} [@var{packages}@dots{}]
+@end example
+
+When @var{packages} is omitted, @command{guix weather} checks the availability
+of substitutes for @emph{all} the packages, or for those specified with
+@option{--manifest}; otherwise it only considers the specified packages.  It
+is also possible to query specific system types with @option{--system}.  The
+available options are listed below.
 
 @table @code
 @item --substitute-urls=@var{urls}

guix/scripts/weather.scm

@@ -252,7 +252,7 @@ are queued~%")
 ;;;
 
 (define (show-help)
-  (display (G_ "Usage: guix weather [OPTIONS]
+  (display (G_ "Usage: guix weather [OPTIONS] [PACKAGES ...]
 Report the availability of substitutes.\n"))
   (display (G_ "
       --substitute-urls=URLS
@@ -469,6 +469,20 @@ SERVER.  Display information for packages with at least THRESHOLD dependents."
 ;;;
 
 (define (guix-weather . args)
+  (define (package-list opts)
+    ;; Return the package list specified by OPTS.
+    (let ((file (assoc-ref opts 'manifest))
+          (base (filter-map (match-lambda
+                              (('argument . spec)
+                               (specification->package spec))
+                              (_
+                               #f))
+                            opts)))
+      (if (and (not file) (null? base))
+          (all-packages)
+          (append base
+                  (if file (load-manifest file) '())))))
+
   (with-error-handling
     (parameterize ((current-terminal-columns (terminal-columns)))
       (let* ((opts     (parse-command-line args %options
@@ -481,10 +495,7 @@ SERVER.  Display information for packages with at least THRESHOLD dependents."
                                           opts)
                          (() (list (%current-system)))
                          (systems systems)))
-             (packages (let ((file (assoc-ref opts 'manifest)))
-                         (if file
-                             (load-manifest file)
-                             (all-packages))))
+             (packages (package-list opts))
              (items    (with-store store
                          (parameterize ((%graft? #f))
                            (concatenate