packages: Add 'hidden-package'.

* guix/packages.scm (hidden-package, hidden-package?): New procedures.
* gnu/packages.scm (fold-packages): Filter out 'hidden-package?'.
* tests/packages.scm ("hidden-package"): New test.

gnu/packages.scm

@@ -200,7 +200,8 @@ same package twice."
    (fold2 (lambda (module result seen)
             (fold2 (lambda (var result seen)
                      (if (and (package? var)
-                              (not (vhash-assq var seen)))
+                              (not (vhash-assq var seen))
+                              (not (hidden-package? var)))
                          (values (proc var result)
                                  (vhash-consq var #t seen))
                          (values result seen)))

guix/packages.scm

@@ -81,6 +81,8 @@
             package-maintainers
             package-properties
             package-location
+            hidden-package
+            hidden-package?
             package-field-location
 
             package-direct-sources
@@ -290,6 +292,19 @@ name of its URI."
                                                        package)
                                                       16)))))
 
+(define (hidden-package p)
+  "Return a \"hidden\" version of P--i.e., one that 'fold-packages' and thus,
+user interfaces, ignores."
+  (package
+    (inherit p)
+    (properties `((hidden? . #t)
+                  ,@(package-properties p)))))
+
+(define (hidden-package? p)
+  "Return true if P is \"hidden\"--i.e., must not be visible to user
+interfaces."
+  (assoc-ref (package-properties p) 'hidden?))
+
 (define (package-field-location package field)
   "Return the source code location of the definition of FIELD for PACKAGE, or
 #f if it could not be determined."

tests/packages.scm

@@ -79,6 +79,10 @@
                     (write
                      (dummy-package "foo" (location #f)))))))
 
+(test-assert "hidden-package"
+  (and (hidden-package? (hidden-package (dummy-package "foo")))
+       (not (hidden-package? (dummy-package "foo")))))
+
 (test-assert "package-field-location"
   (let ()
     (define (goto port line column)