HTTPS support in (guix download)

[Ludovic Courtès]

Hello,

As suggested by Andreas, the (guix download) module now supports
downloads over HTTPS.  For that, it uses the Guile bindings in GnuTLS.

Currently, GnuTLS is only pulled in the build environment for the
download when passed an https URL.  So there’s still the limitation
that, if you pass an http URL, which gets redirected to https, you’ll
get a “missing interface for module (gnutls)” error.  If it’s an problem
in practice, we can add a ‘url-fetch’ parameter to force GnuTLS to be
pulled in when it’s needed (we can’t always have GnuTLS around, because
we don’t want to build it during bootstrap, or maybe we couldn’t.)

In theory, ‘guix-download’ can also work with https URLs.  In practice,
‘guix-download’ uses the “host” Guile (not the one in the chroot), so it
will only work if you have a working GnuTLS in $GUILE_LOAD_PATH.

Comments welcome!

Thanks,
Ludo’.