guix: Split (guix substitutes) from (guix scripts substitute).

guix: Split (guix substitutes) from (guix scripts substitute).

This means there's a module for working with substitutes, rather than all the
code sitting in the script. The need for this can be seen with the weather and
challenge scripts, that now don't have to use code from the substitute script,
but can instead use the substitute module.

The separation here between the actual functionality of the substitute script
and the underlying functionality used both there and elsewhere should make
maintenance easier moving forward.

This commit just moves code, none of the code should have been changed
significantly.

* guix/scripts/substitute.scm (%narinfo-cache-directory, %narinfo-ttl,
%narinfo-negative-ttl, %narinfo-transient-error-ttl, %unreachable-hosts): Move
variables to guix/substitutes.scm.
(narinfo-cache-file, cached-narinfo, cache-narinfo!, narinfo-request,
read-to-eof, call-with-connection-error-handling, fetch-narinfos,
lookup-narinfos, lookup-narinfos/diverse): Move procedures to
guix/substitutes.scm.
* guix/substitutes.scm: New file.
* Makefile.am: Add it.
* guix/narinfo.scm: Remove redundant module.
* guix/scripts/challenge.scm: Change (guix scripts substitute) to (guix
substitutes).
* guix/scripts/weather.scm: Change (guix scripts substitute) to (guix
substitutes).