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
