Permit FFI files in subdirectories #7635

Job	Run time
lint-build	55s
test (stable, x86_64-unknown-linux-gnu)	3m 42s
test-wasm	1m 8s
rustfmt	12s
test (stable, x86_64-unknown-linux-musl)	5m 42s
validate	11s
test (stable, aarch64-unknown-linux-gnu)	1m 59s
test (stable, aarch64-unknown-linux-musl)	2m 24s
test (stable, x86_64-apple-darwin)	5m 55s
test (stable, x86_64-pc-windows-msvc)	5m 25s
test (stable, aarch64-apple-darwin, macos-latest, false, true)	4m 20s
test-projects	1m 52s
	33m 45s

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