Add local_moodlecheck to "composerland"

[stronk7]

This is a proposal to add the required stuff to make this checker composer-compliant so we can proceed with tags/releases and packages @ packagist.

That way, other products using it (say moodle-plugin-ci, say local_ci...), already using composer will be able to define the requirements and proceed to automatically update to new local_moodlecheck versions.

Right now we need to produce new releases in those products because we are using hardcoded commit ids instead of ^x.y versions.

BTW, not 100% sure how composer's lock file affects to this, maybe it also makes things sticky, heh, need to confirm that, because maybe we'll need to perform the new releases always, after all (although they would be more organised).