(#6710) Support relationships for resources defined by other constructs

This adds support for using a variable, resource ref-like string, case
statement, selector, or array/hash access for either side of a
relationship. For example, it is now legal (if unwise) to write the
following:

notify { [a,b,c]: }
$var = Notify[a]
$var -> case 'x' {
  'x': {
    notify { d: }
  }
} ~>
'x' ? {
  'x' => Notify[b]
} <- "Notify[c]"