caching of Tracker code is broken

[AndreasHeger]

... or unreliable:

• “make html” doesn’t seem to see when I’ve updated my tracker code. The program always says “0 trackers changed”, but despite this sometimes does re-runs the tracker, but not always, even if the code has in fact changed. I now always delete the _build directory (_cache itself isn’t enough) before rerunning; it’s a fine workaround but non-intuitive.