exewrapper: prefer HackableMercurial python if availbale
Currently hg.exe will only try to load python27.dll from hg-python
subdir if PYTHONHOME environment variable is not set. I think that
it is better to check whether 'hg-python' subdir exists and load
python from it in that case, regardless of environment. This allows
for reliable approach of distributing Mercurial with its own Python.