Out-of-band downloads

[Ludovic Courtès]

Hello!

Good news: <http://bugs.gnu.org/22774> is fixed!

Previously, all downloads were made by “fixed-output derivations”, which
caused chicken-and-egg problems: sometimes we’d need to build GnuTLS
just to be able to download the source of GnuTLS and its dependencies
over HTTPS.

The fix introduces “out-of-band downloads”: derivations can use a
“built-in builder”, which tells the daemon to perform the build for
themselves.  As a consequence, the derivation does not need to depend on
Guile, GnuTLS, etc.  Instead, the Guile and GnuTLS that are visible to
the daemon are used.

If you use a new daemon, those new derivations will be used.  If you use
a daemon from before today, there won’t be any difference.

Feedback welcome!

Ludo’.