packages: Remove duplicates from package cache.

Previously the same package could appear several times if several
variables were bound to it, as is notably the case for "python"
currently.  This, in turn, would lead to obnoxious "ambiguous package
specification" messages.

* gnu/packages.scm (generate-package-cache)[expand-cache]: Change RESULT
to RESULT+SEEN and adjust accordingly.
Call 'first' on the result of 'fold-module-public-variables*'.
* tests/packages.scm ("fold-available-packages with/without cache"):
Check for lack of duplicates in FROM-CACHE.

gnu/packages.scm

@@ -371,17 +371,21 @@ reducing the memory footprint."
   (define cache-file
     (string-append directory %package-cache-file))
 
-  (define (expand-cache module symbol variable result)
+  (define (expand-cache module symbol variable result+seen)
     (match (false-if-exception (variable-ref variable))
       ((? package? package)
-       (if (hidden-package? package)
-           result
-           (cons `#(,(package-name package)
+       (match result+seen
+         ((result . seen)
+          (if (or (vhash-assq package seen)
+                  (hidden-package? package))
+              (cons result seen)
+              (cons (cons `#(,(package-name package)
                              ,(package-version package)
                              ,(module-name module)
                              ,symbol
                              ,(package-outputs package)
-                    ,(->bool (member (%current-system)
+                             ,(->bool
+                               (member (%current-system)
                                        (package-supported-systems package)))
                              ,(->bool (package-superseded package))
                              ,@(let ((loc (package-location package)))
@@ -390,15 +394,18 @@ reducing the memory footprint."
                                        ,(location-line loc)
                                        ,(location-column loc))
                                      '(#f #f #f))))
-                 result)))
+                          result)
+                    (vhash-consq package #t seen))))))
       (_
-       result)))
+       result+seen)))
 
   (define exp
-    (fold-module-public-variables* expand-cache '()
+    (first
+     (fold-module-public-variables* expand-cache
+                                    (cons '() vlist-null)
                                     (all-modules (%package-module-path)
                                                  #:warn
-                                                warn-about-load-error)))
+                                                 warn-about-load-error))))
 
   (mkdir-p (dirname cache-file))
   (call-with-output-file cache-file

tests/packages.scm

@@ -1037,7 +1037,8 @@
                                                       result))
                                               '()))))))
 
-    (lset= equal? no-cache from-cache)))
+    (and (equal? (delete-duplicates from-cache) from-cache)
+         (lset= equal? no-cache from-cache))))
 
 (test-assert "find-packages-by-name"
   (match (find-packages-by-name "hello")