[2.7] bpo-30450: Pull Windows dependencies from GitHub rather than SVN (GH-1783)...

[2.7] bpo-30450: Pull Windows dependencies from GitHub rather than SVN (GH-1783) (GH-3306)

The Windows build now depends on Python 3.6 to fetch externals, but it
will be downloaded via NuGet (which is downloaded via PowerShell) if it
is not available via `py -3.6`.  This means the only thing that must be
installed on a modern Windows box to do a full build of CPython with all
extensions is Visual Studio.

Cherry-picked from 51599e2bdd10ab77212a7cbb41a13ea70ee13da8, parts of 40a23e88994aca92c83c8e84ab8b8cdc11d7ec54, parts of 68d663cf85d1ac5eaf83482eed39c0a6f8093601, d5cd21d75a27a377f2f9c8370fd8e8c7efaeefb1, and possibly others that I've missed.

Also:

* Rename db -> bsddb for disambiguity

* Update sqlite3 to 3.14.2.0 since it's the version we use on 3.x, and it's simpler to just use it than to also upload the old version to cpython-source-deps

* Add PCbuild/*.ilk to .gitignore