Issue Description:

The following code should cause the type checker to emit an error saying the derivative function is missing @usableFromInline.

Reproducer:

import _Differentiation

public func foo(x: Float) -> Float {
    x
}

@derivative(of: foo)
func fooVJP(x: Float) -> (value: Float, pullback: (Float) -> Float) {
    return (x, { $0 })
}

Current behavior:
If you build it with `swiftc`, the error is correctly produced.
If you build it inside a Swift package with `swift build` (debug configuration), it will build successfully. This is not the right behavior.
If you build it inside a Swift package with `swift build -c release`, it will produce the error as expected.

This is a serious issue that could cause a lot of linker errors on undefined symbols to slip through.