teach xsd.Schema how to merge type maps

[davidalpert]

a new MergeTypes method is added to the xsd.Schema type which merges the types from one xsd.Schema into another provided that the TargetNS values are equivalent.

this MergeTypes method is then used when collecting the map of namespaces to schemas so that when a single schema is spread across multiple files (e.g. when imports are used to tie them together) the types from a namespace are all collected together, regardless of which files they came from.

this allows the resolvePartialTypes method to operate as expected, since following the imports will now merge the types into a single xsd.Schema.