profiles: 'info-dir-file' hook now produces 'dir.LANG' files.

Previously, entries for 'guix.fr.info' would end up in 'dir', above the
'guix.info' entries; consequently, running 'info guix' would actually
open 'guix.fr.info', which was confusing for non-French readers.

* guix/profiles.scm (info-dir-file)[glibc-utf8-locales]: New variable.
[build](info-file-language): New procedure.
(install-info): Use it, to create 'dir.LANG' files.
Set GUIX_LOCPATH.