Better superenv support for Xcode elsewhere

- The Library/ENV/4.3/xcrun shim now respects
  ENV['DEVELOPER_DIR'] instead assuming the location
  of /Applications/Xcode.app/Contents/Developer.
- The env var DEVELOPER_DIR is set if it is not
  already. So, during superenv this var is always set
  and we no longer have to care about people with unset
  or wrongly set xcode-select stuff. This has been
  a major PITA in the past.
- determine_developer_dir (which is used to set
  the DEVELOPER_DIR var) now uses MacOS::Xcode.prefix
  which is proven and very capable and uses splotlight
  correctly.
- Replace (and remove) MacSystem.xcode43_developer_dir
  with Xcode.prefix