[3.6] bpo-30450: Improved logic for obtaining dependencies (#2751)

[3.6] bpo-30450: Improved logic for obtaining dependencies (#2751)

Adds alternate download approach for nuget.exe
Fall back to git.exe if no Python is found. (#2739)
Also check whether git.exe is on PATH if it will be used.
Add support for HOST_PYTHON variable.
Clear internal environment variables used in find_python.bat
Use HOST_PYTHON as the actual Python if it is recent enough.
Adds HOST_PYTHON variable to AppVeyor configuration