substitute: Remove fetch-narinfos use open-connection-for-uri/maybe.

At least by default.  Instead, make the open-connection procedure a parameter,
and make the default guix:open-connection-for-uri.  Do so similarly for
lookup-narinfos and lookup-narinfos/diverse which work towards calling
fetch-narinfos.

This means this code can be moved to a different module, without having
use/move the connection caching code.

* guix/scripts/substitute.scm (fetch-narinfos): Add #:open-connection
argument, and call http-multiple-get with it.
(lookup-narinfos) Add #:open-connection argument, and call fetch-narinfos with
it.
(lookup-narinfos/diverse): Add #:open-connection argument, and call
lookup-narinfos with it.
(process-query): Call lookup-narinfos/diverse with #:open-connection
open-connection-for-uri/maybe.