This moves the installation of shipped source files from large
CMakeLists.txt lists to zig build recursive directory installation.

On my computer a cmake `make install` takes 2.4 seconds even when it has
to do nothing, and prints a lot of unnecessary lines to stdout that say
"up-to-date: [some file it is installing]".

After this commit, the default output of `make` is down to 1
second, and it does not print any junk to stdout. Further, a `make
install` is no longer required and `make` is sufficient.

This closes #2874.

It also closes #2585. `make` now always invokes `zig build` for
installing files and libuserland.a, and zig's own caching system makes
that go fast.