Improve handling of schemas with relative top-level URIs (#237) #310

Job	Run time
package (ubuntu-22.04, clang, clang++, sh)	1m 51s
package (macos-13, clang, clang++, sh)	2m 10s
package (macos-14, clang, clang++, sh)	1m 37s
package (windows-latest, pwsh)	3m 13s
publish	9s
	9m 0s

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