Expose ArgumentParserTestHelpers

Sorry if this is a very naive question or what I'm trying to do is already possible with the current package setup.

I find the `ArgumentParserTestHelpers` target very helpful and it would be great to be able to add it as a dependency for the client's `testTarget`.

As a test, I tried to simply add a [product target for it in a fork](https://github.com/MartinP7r/swift-argument-parser/blob/5936daea708194917d65020187febe402e40c954/Package.swift#L21-L23).

---

> **Note**
> below error was solved by @KS1019's [suggestion](https://github.com/apple/swift-argument-parser/issues/491#issuecomment-1257397121)

However, I keep getting the following error:

```console
product 'ArgumentParserTestHelpers' required by package 'MyApp' target 'MayAppTests' not found in package 'swift-argument-parser'.
```
([Here](https://github.com/MartinP7r/sap-test) is a sample reproducing this error)

---

Best regards,
Martin

Start with Package.swift around the product target at lines 21–23 and compare it with the dependency declaration in the linked sap-test reproduction. Check how ArgumentParserTestHelpers is declared and exposed to a client testTarget. Done means a client can depend on the product without the reported “product ... not found” error.

Written by the indexing model from the issue text.