MDLSITE-2623 Improve the components list for >=2.6

Since 2.6 we can use the core_component class plain and straight
without requiring a complete site to be installed. Let's use it,
that will save some precious minutes on each iteration. Also will
benefit the prechecker (previously always using the moodle-ci-site
list of components).
Merge pull request #11 from stronk7/master

MDLSITE-2623 Improve the components list for >=2.6