Merge overlapping `dependencies` subschemas

[Zac-HD]

In hypothesis_jsonschema._canonicalise.merged(), we should calculate the intersection of dependencies keywords from two schemas.

This is complicated somewhat by the fact that dependencies can be either the bare names, or also schemas. I think if they are of different kinds we can 'promote' the names to a mapping of name to {} (which accepts any value), but it needs some more investigation.

JSONschema draft 2019-09 changed this, but we don't support it yet anyway so that can be left for later.