This provides a number of benefits:

* Significantly reduces the amount of work in win-extras, which I've
always thought was an annoying wart in our build system.

* Provides a consistent 7z executable on all platforms, which can be
picked up by Pkg/BinaryProvider for consistent
unzipping/untarring/ungzipping on all platforms. Note that on Windows,
7z is a repackaged binary from sourceforge and is the "full" 7z, whereas
on all other platforms it is a p7zip build of the standalone 7za
executable with all relevant patches applied. There are some format
capability differences, but for the important things it should be fine.

(cherry picked from commit b6ddd878)