packages: 'specification->package+output' distinguishes "no output specified".

Until now the caller couldn't tell the different between a spec like
"foo:out" and one like "foo".  This change allows users to distinguish
between these two cases.

* gnu/packages.scm (specification->package+output): Disable output
membership test when OUTPUT = #f and SUB-DRV = #f.
* tests/packages.scm ("specification->package+output")
("specification->package+output invalid output")
("specification->package+output no default output")
("specification->package+output invalid output, no default"): New tests.

gnu/packages.scm

@@ -534,14 +534,18 @@ optionally contain a version number and an output name, as in these examples:
   guile@2.0.9:debug
 
 If SPEC does not specify a version number, return the preferred newest
-version; if SPEC does not specify an output, return OUTPUT."
+version; if SPEC does not specify an output, return OUTPUT.
+
+When OUTPUT is false and SPEC does not specify any output, return #f as the
+output."
   (let-values (((name version sub-drv)
                 (package-specification->name+version+output spec output)))
     (match (%find-package spec name version)
       (#f
        (values #f #f))
       (package
-       (if (member sub-drv (package-outputs package))
+       (if (or (and (not output) (not sub-drv))
+               (member sub-drv (package-outputs package)))
            (values package sub-drv)
            (leave (G_ "package `~a' lacks output `~a'~%")
                   (package-full-name package)

tests/packages.scm

@@ -1227,6 +1227,38 @@
     (lambda (key . args)
       key)))
 
+(test-equal "specification->package+output"
+  `((,coreutils "out") (,coreutils "debug"))
+  (list (call-with-values (lambda ()
+                            (specification->package+output "coreutils"))
+          list)
+        (call-with-values (lambda ()
+                            (specification->package+output "coreutils:debug"))
+          list)))
+
+(test-equal "specification->package+output invalid output"
+  'error
+  (catch 'quit
+    (lambda ()
+      (specification->package+output "coreutils:does-not-exist"))
+    (lambda _
+      'error)))
+
+(test-equal "specification->package+output no default output"
+  `(,coreutils #f)
+  (call-with-values
+    (lambda ()
+      (specification->package+output "coreutils" #f))
+    list))
+
+(test-equal "specification->package+output invalid output, no default"
+  'error
+  (catch 'quit
+    (lambda ()
+      (specification->package+output "coreutils:does-not-exist" #f))
+    (lambda _
+      'error)))
+
 (test-equal "find-package-locations"
   (map (lambda (package)
          (cons (package-version package)