Group @testable imports at the bottom, per public/rules-swift's swiftformat. A
behavior-preserving import-ordering reformat across 252 files (mostly tests).
--trailingclosures was deliberately not adopted (cosmetic, and swiftformat flags
it as caution-worthy).
A simpleType nested inside a restriction/list/union/element/attribute is the
anonymous localSimpleType (no name), and a nested attributeGroup is always a
reference; named simpleType and attributeGroup definitions are top-level only (a
direct child of schema or redefine). A nested definition declaring a name was
accepted; it is now rejected. A nested anonymous simpleType, an attributeGroup
reference, and a top-level named definition are unaffected.
XSTS: invalid schemas accepted 403 to 394, no other bucket raised.
