* guix/build-system/julia.scm: Remove unused imports. (lower)[julia]: Default to (DEFAULT-JULIA).
* guix/build-system/julia.scm: Fix invalid module reference.
* guix/build/julia-build-system.scm: New file. * guix/build-system/julia.scm: New file. * Makefile.am (MODULES): Add them. * doc/guix.texi (Build Systems): Document julia-build-system. Signed-off-by: Julien Lepiller <julien@lepiller.eu>