Force the installation of watchman and any dependencies in the macOS
installation script. This helps get around issues whereby dependencies
of watchman (looking at you Python) complain during installation that
some extra tools were not linked (unless you specify --force), and the
script fails overall.

Also set ownership of several Homebrew directories which may be broken,
and remove any errant 2to3 Python binaries that may exist in
/usr/local/bin, which would cause linking issues when installing
watchman's Python 3 dependency.