When includes are nested more than two levels deep, the base URI is not preserved #28
Description
This is the root cause of xmlcalabash/xmlcalabash3#321
If you build a document from the bottom up, as XInclude does, and you preserve the base URI, as XInclude does, a problem arises when nesting is more than two levels deep. Suppose A includes B includes C. When you process C, you record its base URI and it gets incorporated into the result returned for B. That's fine. Now when you process B, you record its base URI and it gets incorporated into A. Therein lies the rub. C is already included in B. When B gets included in A, the base URI associated with B gets applied to all of B, including C within it. This isn't a problem when fixupXmlBase is true because the xml:base attribute added at each level will override the base URI being applied.
But when fixupXmlBase is false, there's nothing to override the base URI.
This isn't a practically solvable problem in Saxon, so we'll have to do it here.