don't use new name rack if old name rack is a dir

Everything that used HOMEBREW_CELLAR/canonical_name
could point to something that doesn't exist because
loader_for tries to load new name formula if no old
name found. However there can be software installed
from path with the same name that renamed formulae
had and we still need to link/unlink/uninstall etc
that software. The solution is Formulary#to_rack
method that returns rack for given name.
- Add Formulary#to_rack
- Update ARGV.kegs
- Update cmd/switch