Simplify conditions for superenv activation

`MacOS::Xcode.without_clt? && MacOS.sdk_path.nil?` should never be true.

In its earliest form, this would raise a bare RuntimeError in an effort
to have the bug reported. Later, it was changed to silently disable
superenv. But we don't want to do that. If there's a bug, or the user's
system is misconfigured, we want to know, so that we can fix the bug, or
the user can fix their system. So let's remove the condition.
Only directories are candidates for superenv bin
Trust xcrun now that we error out early for bad xcode-select paths
Move xctoolchain_path to Xcode module, where it belongs
Rename Xcode.folder to MacOS.active_developer_dir

This reflects what Apple calls it in more recent versions of Xcode/OS X.
Simplify dev_tools_path by checking active_developer_dir first
add TODOs, questions