MDLSITE-2885 Install composer into alt directory

This script will automatically install and update
the composer stuff as specified by Moodle's composer.json
keeping it out from the dirroot/vendor and installing it
into another alternative path, to avoid it being cleaned
for every build.

Any other script requiring composer stuff (phpunit, behat for now)
will have to execute this script and, magically, the executables
will be available for them.
MDLSITE-2885 Run phpunit form composer.
MDLSITE-2885 Link to dirroot/vendor

We have some built-in dependencies in our tools requiring vendor
to be within dirroot, so soft-linking out custom vendor will
provide that, grrr.
MDLSITE-2885 Specify the exact php executable to use.

In sites with multiple PHP versions installed, composer
may end picking the incorrect executable. Let's always
prioritize the one we use in CI ($phpcmd) via PATH.
MDLSITE-2885 Always recreate the link.

This may help in situations where a new clone is based
in a previous branch or something is renamed. Not usual
in production, but in development environments.
MDLSITE-2885 Add information along the progress