Clarify, simplify, or rewrite IsExposed(member) algorithm

[glen-84]

Some initial questions:

1. • Let {members} be a list of all members across all subgraphs with the same
     coordinate and kind as {member}
   • If any {members} is marked with @internal
     • return false

   According to the spec, @internal can only be applied to field definitions. The spec for IsExposed(member) seems to handle all member types. Can @internal be applied to other member types, or is IsExposed(member) over-specced?

2. • If {member} is InputField
   • Let {type} be any input object type that {member} is declared on

   What does "any input object type" mean? The input field would only have one parent input object type, right?

3. There is no spec for IsExposed(type). If we assume that it just checks for @internal on the input object type, then this also contradicts the specification of the @internal directive, that can only be applied to field definitions.