Support directives declared stitched schemas

[nigel-sampson]

This is the step in adding support for directives declared in a stitched schema to be included in the resulting merged schema. I don't think this is the final bit of work in this area but wanted to create the PR now in order to start some of the discussion.

Directives work a little differently than a lot of the other types so I'm not sure if merging can work in the same way.

Some of the guiding assumptions I made:

1. Directives type declarations cannot themselves have directives applied to them. This means we cannot use the @source directive to support type name rewriting.
2. Without type name rewriting it doesn't make sense to try and generate unique names for directives that cannot be merged. This PR throws an exception for this scenario, if another avenue for storing the source directive name emerges then this should be possible.
3. Given this closed nature of directive merging I haven't tried to add any extension points for custom directive merge handlers.

[michaelstaib]

I will look at this tonight ...

[michaelstaib]

This looks good to me :)