gnu: Switch to 'with-imported-modules'.

gnu: Switch to 'with-imported-modules'.

* gnu/services.scm (directory-union): Use 'with-imported-modules'
instead of the '#:modules' argument of 'computed-file'.
* gnu/services/base.scm (udev-rules-union): Likewise.
* gnu/services/dbus.scm (system-service-directory): Likewise.
* gnu/services/desktop.scm (wrapped-dbus-service):
(polkit-directory): Likewise.
* gnu/services/networking.scm (tor-configuration->torrc): Likewise.
* gnu/services/xorg.scm (xorg-configuration-directory): Likewise.
* gnu/system/install.scm (self-contained-tarball): Likewise.
* gnu/system/linux-container.scm (container-script): Likewise.
* gnu/system/linux-initrd.scm (expression->initrd): Likewise, and
remove #:modules parameter.
(flat-linux-module-directory): Use 'with-imported-modules'.
(base-initrd): Likewise.
* gnu/system/locale.scm (locale-directory): Likewise.
* gnu/system/shadow.scm (default-skeletons): Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Likewise.
* gnu/tests/base.scm (run-basic-test): Likewise.
* gnu/tests/install.scm (run-install): Likewise.
* doc/guix.texi (Initial RAM Disk): Update 'expression->initrd'
documentation.