Resolve relative XInclude system ids via `EntityResolver2`

`ConfigurationSourceResolver` resolved `xi:include` targets through the
`EntityResolver` interface, whose `resolveEntity(publicId, systemId)` lacks the
base URI, so a parser that passes an unresolved, relative `systemId` (such as
the target of a nested `xi:include`) could not be satisfied. Implement
`EntityResolver2` so the resolver receives the base URI and resolves the
`systemId` against it, the same way `resolveResource` already does for schema
imports.

Assisted-By: Claude Opus 4.8 <noreply@anthropic.com>