Laid the foundation for supporting virtual environments and docker instances

The big change in this release is the way the destination prefix is computed: mm does not append a target dependent tag to it before passing it on to make. This gives the user complete control over the destination directory and simplifies using mm in virtual environments and docker instances. More on this to come.

mm now pulls pyre version 1.11.2 from GitHub when it cannot find a usable local installation.

Added support for aarch64

Full Changelog: v4.2.2...v4.3.0