To prevent bugs and complications, we should use autoloader during piwik.php requests.
NOTE: this requires that first we PSR4 autoloader: #4074
Because our current autoloader is not good enough and would be too slow for tracker.
I added the autoloader here: https://github.com/piwik/piwik/commit/0e6ec5d9a0d3405ea32001ab57bf2a293c462f7b
At the moment most classes are loaded manually anyway. So only a very few classes will be loaded by the autloader anyway. And after the change to the autoloader a few weeks ago the PSR4 autoloader should be much faster if at all.