guix build: Allow directories to be passed to --with-source.

* guix/scripts/build.scm (package-with-source)[tarball-base-name]: Gracefully
  handle file names that lack an extension.
  Pass #:recursive? #t to 'download-to-store'.
* guix/download.scm (download-to-store): Add #:recursive? parameter and pass
  it to 'add-to-store'.
* doc/guix.texi (Invoking guix build): Add an example of --with-source with a
  directory.