demandimport: instrument python 2 code with trace events This causes the evaluation of an import in Python 3 to emit some trace data. There's some interesting wrinkles in here, like the fact that before we even hit dispatch we've demand-imported `sys` several times, despite the fact that `sys` was already fully loaded as one of the first few statements in the `hg` script. I don't think that's actually costing us a ton of performance, but it's probably something we should investigate fixing some day. Differential Revision: https://phab.mercurial-scm.org/D4347