gnu: commencement: mes-boot: Use new package style.

* gnu/packages/commencement.scm (mes-boot)[arguments]: Use gexp for modify
phases.

gnu/packages/commencement.scm

@@ -289,9 +289,9 @@ pure Scheme to Tar and decompression in one easy step.")
            #:builder
            #~(begin
                (use-modules (guix build utils))
-               (let ((source (assoc-ref %build-inputs "source"))
+               (let ((source #$(package-source this-package))
-                     (tar (assoc-ref %build-inputs "bootar"))
+                     (tar #$(this-package-native-input "bootar"))
-                     (out (assoc-ref %outputs "out")))
+                     (out #$output))
                  (setenv "PATH" (string-append tar "/bin:"))
                  (invoke "tar" "xvf" source)
                  (mkdir-p out)
@@ -365,13 +365,14 @@ hex1, hex2, M1, and M2-Planet.")
         #:builder
         #~(begin
             (use-modules (guix build utils))
-            (let* ((bootstrap-seeds (assoc-ref %build-inputs "bootstrap-seeds"))
+            (let* ((bootstrap-seeds #$(this-package-native-input
-                   (source (assoc-ref %build-inputs "source"))
+                                       "bootstrap-seeds"))
-                   (tar (assoc-ref %build-inputs "bootar"))
+                   (source #$(package-source this-package))
-                   (bash (assoc-ref %build-inputs "bash"))
+                   (tar #$(this-package-native-input "bootar"))
-                   (coreutils (assoc-ref %build-inputs "coreutils"))
+                   (bash #$(this-package-native-input "bash"))
-                   (guile (assoc-ref %build-inputs "guile"))
+                   (coreutils #$(this-package-native-input "coreutils"))
-                   (out (assoc-ref %outputs "out"))
+                   (guile #$(this-package-input "guile"))
+                   (out #$output)
                    (bindir (string-append out "/bin"))
                    (target (or #$(%current-target-system)
                                #$(%current-system)))
@@ -444,22 +445,22 @@ MesCC-Tools), and finally M2-Planet.")
                                  (snippet #f))))
        ,@(%boot-gash-inputs)))
     (arguments
-     `(#:implicit-inputs? #f
+     (list
+      #:implicit-inputs? #f
       #:tests? #f
-       #:guile ,%bootstrap-guile
+      #:guile %bootstrap-guile
       #:strip-binaries? #f              ;no strip yet
       #:phases
-       (modify-phases %standard-phases
+      #~(modify-phases %standard-phases
           (add-after 'unpack 'unpack-seeds
             (lambda _
-             (let ((nyacc-source (assoc-ref %build-inputs "nyacc-source")))
+              (let ((nyacc-source #$(this-package-native-input "nyacc-source")))
                 (with-directory-excursion ".."
                   (invoke "tar" "-xvf" nyacc-source)))))
           (replace 'configure
             (lambda* (#:key inputs outputs #:allow-other-keys)
-             (let ((out (assoc-ref outputs "out"))
+              (let ((out #$output)
-                   (gash (assoc-ref inputs "bash"))
+                    (gash #$(this-package-native-input "bash"))
-                   (mes (assoc-ref inputs "mes"))
                     (dir (with-directory-excursion ".." (getcwd))))
                 (setenv "GUILE_LOAD_PATH" (string-append
                                            dir "/nyacc-1.00.2/module"))
@@ -479,7 +480,7 @@ MesCC-Tools), and finally M2-Planet.")
                                 space cmd "\n")))
               (invoke "gash" "install.sh")
               ;; Keep ASCII output, for friendlier comparison and bisection
-             (let* ((out (assoc-ref outputs "out"))
+              (let* ((out #$output)
                      (cache (string-append out "/lib/cache")))
                 (define (objects-in-dir dir)
                   (find-files dir