MSVC Builds: Use PythonDir instead of PythonPath

If someone is building using MSBuild, we want to make sure that we do
not use an envvar that would also determine where the Python modules are
loaded.

Also, allow different Python paths depending on the Visual Studio
version that is being used for Visual Studio 201x builds, to better suit
the version of  Visual Studio that is used to build the official
releases of Python, which means the default is as follows:

-Visual Studio 2010, 2012, 2013: Python 3.4.x, which is built with 2010
-Visual Studio 2015 and 2017: Python 3.6.x, which is built with 2015.

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