XSD 1.0 allows a complexType's content to be simpleContent, or complexContent,
or a model group followed by attributes, never a mix. The allowedChildren
set-membership check permitted illegal combinations, so a complexType with two
simpleContent, simpleContent beside a sequence, or two model groups was accepted.
complexTypeContentErrors now enforces, over the complexType's direct children:
at most one of simpleContent/complexContent; when one is present, no direct model
group and no direct attribute (those belong inside the extension/restriction);
and at most one model group. Only direct children are examined, so attributes
nested inside a derivation are not flagged.
XSTS: invalid schemas accepted 1051 to 997 (-54); valid schemas rejected held at
72; instance buckets unchanged. Also folds in a behavior-identical lint cleanup
of the reference-walk external-definition check.