installer: Move everything to the build side.

installer: Move everything to the build side.

* gnu/installer.scm: Rename to ...
* gnu/installer/record.scm: ... this.
* gnu/installer/build-installer.scm: Move everything to the build side and
rename to gnu/installer.scm.
* gnu/installer/newt.scm: Remove all the gexps and add depencies to newt
modules as this code will only be used on the build side by now.
* gnu/local.mk (GNU_SYSTEM_MODULES): Adapt it,
(dist_installer_DATA): New rule to install installer's aux-files.
* gnu/system/install.scm (%installation-services): Use only
'installer-program' from (gnu installer). The installer is now choosen on the
build side.
* guix/self.scm (*system-modules*): Restore previous behaviour and add all
installer files to #:extra-files field of the scheme-node.
* po/guix/POTFILES.in: Adapt it.