A relative system identifier declared inside an external entity
resolves against that entity's own URI (RFC 3986, the C14N resolver):
ExternalID gains base and resolvedSystemID, the scanner threads the
current entity's URI through external-subset and PE scans, and
resolvers receive the merged identifier. Clears eduni E18; the eduni
baseline now holds only the grapheme-lexing class (#135).
Closes#138
