fix: handle shallow export resource URIs in ExportGeneratorX

[joaodinissf]

Summary

• fix the pre-existing IllegalArgumentException in ExportGeneratorX for shallow platform:/resource/<project>/<file>.export URIs
• keep the existing package derivation for resources under src/<java package>/...
• fall back to a sanitized project-name package for root-level .export files
• add a regression test for the root-level test resource URI shape

Verification

• env WORKSPACE=$(pwd) mvn -f ./ddk-parent/pom.xml -pl :ddk-target,:com.avaloq.tools.ddk.test.core,:com.avaloq.tools.ddk.xtext.test.core,:com.avaloq.tools.ddk.xtext.export,:com.avaloq.tools.ddk.xtext.export.ui,:com.avaloq.tools.ddk.xtext.export.test -am verify -DfailIfNoTests=false

Limitations

• this verifies compilation and packaging of the affected bundles, but com.avaloq.tools.ddk.xtext.export.test does not execute tests in this reactor because it inherits tycho-surefire-plugin skip=true from ddk-parent
• running the actual Tycho UI test runner is currently blocked by an unrelated existing master compile failure in com.avaloq.tools.ddk.xtext.test/src/com/avaloq/tools/ddk/xtext/naming/QualifiedNameSegmentTreeLookupTest.java