Compiling for watchOS 4.0, but module 'SwiftyStoreKit' has a minimum deployment target of watchOS 6.2

## Bug Report
I want to use SwiftyStoreKit in an watchOS app with deployment target of watchOS 4.0.
Prior watchOS 6.2 I cannot use the Framework, since the API was introduced with wOS 6.2.

Thats why I've imported the framework optionally and marked the class to use the framework with `@available(watchOS 6.2, *)`.

However, I cannot compile the project:
`Compiling for watchOS 4.0, but module 'SwiftyStoreKit' has a minimum deployment target of watchOS 6.2`.

I've tried using
```swift
#if canImport(SwiftyStoreKit)
import SwiftyStoreKit
#endif
```

…but that didn't solve my problem.

Any ideas? It seems like this has been fixed in #562 but somehow not for me?
(I am using version `0.16.1`).