Take a step toward complete BB domination (JuliaLang/julia#33125)

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.

LICENSE.md

@@ -77,11 +77,10 @@ Julia's build process uses the following external tools:
 - [PATCHELF](https://nixos.org/patchelf.html)
 - [OBJCONV](https://www.agner.org/optimize/#objconv)
 
-Julia bundles the following external programs and libraries on some platforms:
+Julia bundles the following external programs and libraries:
 
 - [7-Zip](https://www.7-zip.org/license.txt)
 - [ZLIB](https://zlib.net/zlib_license.html)
-- [LIBEXPAT](http://expat.cvs.sourceforge.net/viewvc/expat/expat/README)
 
 On some platforms, distributions of Julia contain SSL certificate authority certificates,
 released under the [Mozilla Public License](https://en.wikipedia.org/wiki/Mozilla_Public_License).