Fix invalid xs:import of fmi3Annotation.xsd

[hubertus65]

Summary

maHarmonizedSpecificationExperiments.xsd pulled in fmi3Annotation.xsd via xs:import with no namespace attribute:

<xs:import schemaLocation="https://raw.githubusercontent.com/modelica/fmi-standard/main/schema/fmi3Annotation.xsd"/>

Neither this schema nor fmi3Annotation.xsd declares a targetNamespace — both are in "no namespace". Per XSD 1.0 (§4.2.3), xs:import is for pulling in components from a different namespace than the importing schema; merging components from a schema with the same (or absent) target namespace is what xs:include is for. xs:import without a namespace attribute, used by a schema that itself has no target namespace, is not spec-conformant.

Lenient validators (libxml2/xmllint, many IDEs) tolerate this, which is presumably why it went unnoticed, but strict tooling rejects the schema outright — e.g. the Python xmlschema library fails to even load it:

XMLSchemaParseError: if the 'namespace' attribute is not present on the import statement then the imported schema must have a 'targetNamespace'

Fix: xs:import → xs:include for this one line (xs:include takes no namespace attribute).

Closes #7

Test plan

• Confirmed the schema and example manifest validate successfully with the strict Python xmlschema library after the change (previously failed to even load the schema)

[AnHeuermann]

I recommend to merge #10 before merging this to see in the CI that the XML is valid with this fix.