build-system/python: Memoize the results of 'package-with-python2'.

Fixes <http://bugs.gnu.org/21675>.
Reported by Cyril Roelandt <tipecaml@gmail.com>.

Before this change, the command:

  guix build python2-oslo.utils -n --no-substitutes

would take 17.5s.  After, it is down to 2.9s.
Likewise, the command:

  guix graph python2-bandit | grep python2-setuptools.*Helve| wc -l

would return 412 nodes before, all functionally equivalent, and returns
a single one now.

* guix/build-system/python.scm (package-with-explicit-python): Remove
  'p' parameter.  Change to return a one-argument memoizing procedure.
  (package-with-python2): Adjust accordingly.

guix/build-system/python.scm

@@ -56,19 +56,25 @@ (define (default-python2)
   (let ((python (resolve-interface '(gnu packages python))))
     (module-ref python 'python-2)))
 
-(define (package-with-explicit-python p python old-prefix new-prefix)
+(define (package-with-explicit-python python old-prefix new-prefix)
-  "Create a package with the same fields as P, which is assumed to use
+  "Return a procedure of one argument, P.  The procedure creates a package with
-PYTHON-BUILD-SYSTEM, such that it is compiled with PYTHON instead.  The
+the same fields as P, which is assumed to use PYTHON-BUILD-SYSTEM, such that
-inputs are changed recursively accordingly.  If the name of P starts with
+it is compiled with PYTHON instead.  The inputs are changed recursively
-OLD-PREFIX, this is replaced by NEW-PREFIX; otherwise, NEW-PREFIX is
+accordingly.  If the name of P starts with OLD-PREFIX, this is replaced by
-prepended to the name."
+NEW-PREFIX; otherwise, NEW-PREFIX is prepended to the name."
+  (define transform
+    ;; Memoize the transformations.  Failing to do that, we would build a huge
+    ;; object graph with lots of duplicates, which in turns prevents us from
+    ;; benefiting from memoization in 'package-derivation'.
+    (memoize                                      ;FIXME: use 'eq?'
+     (lambda (p)
        (let* ((rewrite-if-package
                (lambda (content)
-            ;; CONTENT may be a file name, in which case it is returned, or a
+                 ;; CONTENT may be a file name, in which case it is returned,
-            ;; package, which is rewritten with the new PYTHON and NEW-PREFIX.
+                 ;; or a package, which is rewritten with the new PYTHON and
+                 ;; NEW-PREFIX.
                  (if (package? content)
-                (package-with-explicit-python content python
+                     (transform content)
-                                              old-prefix new-prefix)
                      content)))
               (rewrite
                (match-lambda
@@ -76,11 +82,13 @@ (define (package-with-explicit-python p python old-prefix new-prefix)
                   (append (list name (rewrite-if-package content)) rest)))))
 
          (if (eq? (package-build-system p) python-build-system)
-        (package (inherit p)
+             (package
+               (inherit p)
                (name (let ((name (package-name p)))
                        (string-append new-prefix
                                       (if (string-prefix? old-prefix name)
-                                     (substring name (string-length old-prefix))
+                                          (substring name
+                                                     (string-length old-prefix))
                                           name))))
                (arguments
                 (let ((arguments (package-arguments p))
@@ -88,18 +96,21 @@ (define (package-with-explicit-python p python old-prefix new-prefix)
                                      (force python)
                                      python)))
                   (if (member #:python arguments)
-                 (substitute-keyword-arguments arguments ((#:python p) python))
+                      (substitute-keyword-arguments arguments
+                        ((#:python p) python))
                       (append arguments `(#:python ,python)))))
                (inputs (map rewrite (package-inputs p)))
                (propagated-inputs (map rewrite (package-propagated-inputs p)))
                (native-inputs (map rewrite (package-native-inputs p))))
-        p)))
+             p)))))
+
+  transform)
 
 (define package-with-python2
   ;; Note: delay call to 'default-python2' until after the 'arguments' field
   ;; of packages is accessed to avoid a circular dependency when evaluating
   ;; the top-level of (gnu packages python).
-  (cut package-with-explicit-python <> (delay (default-python2))
+  (package-with-explicit-python (delay (default-python2))
                                 "python-" "python2-"))
 
 (define* (lower name