bugfix: handle verus-aware ImplItems in external Impls #6461

Job	Run time
smoke-test-and-release-windows	7m 18s
smoke-test-and-release-macos-x86	14m 35s
smoke-test-and-release-linux	6m 18s
fmt	1m 13s
test-and-release-macos	13m 25s
test-and-release-macos (record-history)	3m 40s
test-and-release-macos (no-std)	3m 8s
test-and-release-macos (no-alloc)	3m 0s
test-and-release-macos (singular)	2m 56s
test-and-release-macos (cvc5)	2m 50s
release	15s
	58m 38s

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