Ruleset should be made available for plugins to reference

In https://github.com/WordPress/performance/issues/2043 we found out that the Performance Lab plugin is failing some PHPCS checks from PCP.

To fix them I had to find what the specific PHPCS rules which were used and incorporate them into our ruleset. I wanted to install the PHPCS rules via composer on Packagist, but I had to install them via WPackagist. It would be nice if this installation via WPackagist wasn't required.

Then to actually incorporate the rules, I had to extract the rules from the PCP ruleset into a new ruleset XML file and include it our ruleset. I couldn't directly reference your ruleset because it includes configuration we don't want, for example the PHP 5.2+ compatibility. It would be nice if the core rules for PCP were put into a separate base ruleset which we could reference, and then you could have your ruleset reference it as well for the PCP process.

See https://github.com/WordPress/performance/pull/2048 for what I've had to do to incorporate the PCP ruleset.