php8: xml modules affect others

php8: xml modules affect others

As with gettext modules described in #28078 and #28075, xml and dom
related module selection affects the dependencies of other packages.

Therefore, we invert the dependency logic:

PHP8_LIBXML and PHP8_DOM are are enabled by default and packages
which depend on libxml2 and --enable-dom=shared are not shown (and
the related configure args are disabled) if the config options are
not enabled.

Signed-off-by: Daniel F. Dickinson <dfdpublic@wildtechgarden.ca>