build-system/julia: Don't rely on file name to set module name.

* guix/build/julia-build-system.scm (project.toml->name): New procedure.
  (precompile, check, julia-build): Accept new key argument #:julia-package-name.
* guix/build-system/julia.scm (julia-build): ... add it.
* doc/guix.texi (julia-build-system): Update julia-package-name accordingly.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>