Automatic rankmirrors on install

[jeremyd2019]

I've been mulling this over for a while, and msys2/MINGW-packages#7925 kicked it back to the foreground again.

I had been considering an install script or hook to automatically run rankmirrors on mirrorlists. As in, change pacman-mirrors to install mirrorlist.*.dist, and write mirrorlist.* as output of running rankmirrors on the .dist version.

But, I noticed that pacman-mirrors is a 'core' package, which has me concerned that something like #2295/#2335 would happen trying to run any such script.

Also, any such auto-rankmirrors script should also be included in /etc/post-install for fresh installs.