Use singletons-base, not singletons, as the extra_package

Previously, the `singletons` package included all of what is now part of
`singletons-base`. To ensure that we are still testing this code on CI, make
`singletons-base` the `extra_package`.

ci/config.sh

@@ -98,7 +98,7 @@ extra_package criterion
 extra_package scotty
 extra_package generic-lens 2.2.0.0
 extra_package microstache
-extra_package singletons 3.0
+extra_package singletons-base 3.0
 extra_package servant 0.18.3
 extra_package hgmp
 extra_package Agda 2.6.1.3