packages: Refer to the native tools when handling sources and downloads.

* guix/packages.scm (patch-and-repack)[build]: Change most #$ to #+.
* guix/cvs-download.scm (cvs-fetch)[build]: Likewise.
* guix/download.scm (url-fetch)[builder]: Likewise.
* guix/git-download.scm (git-fetch)[build]: Likewise.
* guix/svn-download.scm (svn-fetch)[build]: Likewise.