add pacman to known providers

yeah msys2 is out in their own world, they may nominally be using a port of pacman but their repository organization and possibly naming of packages is completely different since there are 3 set of packages - msys which is using the cygwin-fork posix compatibility dll, mingw32 and mingw64 which don't. at the moment we can't do anything involving the package manager while julia is started up inside msys2, that may change with libgit, but i'd be hesitant to rely on things that only work when julia is started within msys2.