download: Add HTTPS support.

* guix/build/download.scm: Autoload (gnutls).
  (tls-wrap): New procedure.
  (open-connection-for-uri): Add support for `https'.  Wrap the socket
  with `tls-wrap' in that case.
  (url-fetch): Add `https'.
* guix/download.scm (gnutls-derivation): New procedure.
  (url-fetch)[need-gnutls?]: New variable.
  Call `gnutls-derivation' when NEED-GNUTLS? is true, and add its output
  to the `GUILE_LOAD_PATH' env. var. in that case.