Make pkgstyles provide default distro aliases such as `deb` and `rpm`

pkgstyles already contain SUPPORTED_DISTROS, use these to automatically provide distro aliases per pkgstyle so that this is always possible:

{%- if distro.match('deb') %}
Depends: iputils-ping, ca-certificates
{%- elif distro.match('rpm') %}   
Depends: iputils
{%- endif %}