Better default namespace resolution (#406)

Features/Bugfixes

Default namespaces for Queries and Mutations are now cleanly applied
to the @field directive.

Breaking

If no resolver was found for a field, Lighthouse throws an Exception. This will only
help uncover bugs and help users to fix mistakes in their schema definition.

Internals that concern how field resolvers are built were slightly changed, only users that
reach far into the internal methods for defining custom directives might be affected.