run-tests: factor out _checkhglib() to check import path of 'mercurial'. - rename _hgpath() to _gethgpath() and move it down next to _checkhglib() - change _gethgpath() so it caches its result: replaces global 'hgpkg' that anyone can use with global '_hgpath' that is only for _gethgpath() - no need to pass 'expecthg' to runchildren() or runtests() anymore Also: do not change global PYTHON when doing coverage; this seems to be unnecessary, since we create a dummy 'hg' script that handles coverage for us, and it made things brittle. (E.g. the rest of this patch makes the call to _hgpath() come later, and it was broken by enabling coverage.)