Split alerts/alerts.libsonnet into one file per collector under alerts/,
like kubernetes-mixin. Each rule is a plain Prometheus alerting rule with
the PromQL written out in an expr block and %(name)s placeholders for the
_config values; lib/alerts.libsonnet keeps the collector and per-alert
gating and keep_firing_for. The pagefile, diskdrive and license alerts
get groups of their own so that one file is one group. The rendered
default rules are unchanged apart from expr block scalars.
Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>