/usr/share/gnome-software/modulesets.d/core.xml is in gnome-software-common 3.20.1+git20160420.1.ca63436.ubuntu-xenial-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | <moduleset name="core">
<module type="pkgname">glib2</module>
<module type="pkgname">glibc</module>
<module type="pkgname">gtk3</module>
<module type="pkgname">kernel</module>
</moduleset>
|