gnu: commencement: Do not graft early bootstrap packages.

By definition, these packages are not depended on at run time by any of
the packages we use.  Thus it does not make sense to inherit grafts.
Furthermore, those grafts would often lead to extra overhead for users
who would end up downloading those "-boot0" packages just to build
package replacements that are in fact not going to be used.

This reverts parts of f1597427f2 and
ce27857f71.

Reported by Christopher Baines at
<https://lists.gnu.org/archive/html/guix-devel/2017-12/msg00058.html>.

* gnu/packages/commencement.scm (file-boot0, binutils-boot0): Use plain
'inherit' instead of 'package/inherit'.

gnu/packages/commencement.scm

@@ -113,7 +113,8 @@
 
 (define file-boot0
   (package-with-bootstrap-guile
-   (package-with-explicit-inputs (package/inherit file
+   (package-with-explicit-inputs (package
+                                   (inherit file)
                                    (name "file-boot0"))
                                  `(("make" ,gnu-make-boot0)
                                    ,@%bootstrap-inputs)
@@ -140,7 +141,7 @@
 
 (define binutils-boot0
   (package-with-bootstrap-guile
-   (package/inherit binutils
+   (package (inherit binutils)
      (name "binutils-cross-boot0")
      (arguments
       `(#:guile ,%bootstrap-guile