build-systems: gnu: Export %default-gnu-imported-modules and %default-gnu-modules.

Until now users would have to cargo cult or inspect the private
%default-modules variable of (guix build-systems gnu) to discover which
modules to include when extending the used modules via the #:modules argument.

The renaming was automated via the command:

  $ git grep -l %gnu-build-system-modules
    | xargs sed 's/%gnu-build-system-modules/%default-gnu-imported-modules/' -i

* guix/build-system/gnu.scm (%gnu-build-system-modules): Rename to...
(%default-gnu-imported-modules): ... this.
(%default-modules): Rename to...
(%default-gnu-modules): ... this.  Export.
(dist-package, gnu-build, gnu-cross-build): Adjust accordingly.

Change-Id: Idef307fff13cb76f3182d782b26e1cd3a5c757ee

gnu/packages/version-control.scm

@@ -263,7 +263,7 @@ Python 3.3 and later, rather than on Python 2.")
                   (ice-9 format)
                   (ice-9 textual-ports)
                   ((guix build gnu-build-system) #:prefix gnu:)
-                  ,@%gnu-build-system-modules)
+                  ,@%default-gnu-modules)
       ;; Make sure the full bash does not end up in the final closure.
       #:disallowed-references (list bash perl)
       #:test-target "test"
@@ -2102,7 +2102,7 @@ control to Git repositories.")
     (build-system gnu-build-system)
     (arguments
      `(#:imported-modules ((guix build guile-build-system)
-                           ,@%gnu-build-system-modules)
+                           ,@%default-gnu-imported-modules)
        #:make-flags (list "GUILE_AUTO_COMPILE=0")
        #:phases
        (modify-phases %standard-phases
@@ -4093,7 +4093,7 @@ TkDiff is included for browsing and merging your changes.")
      (list
       #:tests? #f                       ;No tests.
       #:imported-modules
-      `(,@%gnu-build-system-modules
+      `(,@%default-gnu-imported-modules
         (guix build python-build-system))
       #:modules
       '((guix build gnu-build-system)