An xs:all group may only be the whole content model of a complex type. A direct
all nested in a sequence/choice was already rejected; this closes the case where
an all is reached through a <group ref> to a named group whose content is an all
and that reference appears inside a compositor.
The all-group pool is qualified by the defining schema's target namespace, and a
reference by its prefix binding, so an imported or redefined same-named group is
not confused with a local one (two over-rejections found by an adversarial
critic: a redefined all-group whose content is overridden, and an imported
all-group shadowing a local non-all group; both fixed and tested). A group
redefined to non-all content is exempt.
XSTS: invalid schemas accepted 481 to 480, no other bucket raised. Disclosed
under-rejections: a redefine that keeps the group an all, and cross-namespace
misplacement of an imported all-group.