As the title says. pacman -Q lists only name and version; pacman -Qi does have a "Packager" field, but i think it's not the same thing; pacman -Qsseems to be what i want (if local means "all installed packages atm") but it's all prefixed by local/ instead of repo name like mingw32/ which is what i want.
pacman does not store the source repository of installed packages, that is why it does not have an option to show that information. You probably have to write some script to find possible repositories for each package.
Maybe the expac tool already has something, but I cannot check at the moment.
etc, this will give a list of packages from that repo, because msys2 uses prefixes to define the packages and not have conflicts. A script that just make these commands might work, packages with no prefix are from the msys repo.