schemaLocation remapping when validating against a schema directory

[bworrell]

When validating a document using --schema-dir or the schemas parameter, the underlying libxml2 schema parsing engine still attempts to resolve remote xs:import and xs:include schemas. This can hang the validator when the system is offline.

We should look into remapping xs:include and xs:import statements to localized schemas found within the input schema directory. This would require a pre-parse of the schemas and the creation of a localized, tmp set of modified schemas to validate against.

This gets tricky when you factor in xs:include, because multiple schemas define the same targetNamespace.

This may also be something we just don't want to do, as we'd technically be validating against a modification of the input schemas and not the input schemas themselves. This could potentially result in undesired behaviors and false positives/negatives. We need to consider if this is okay or not.