Change the visibility of the derived module and of the AsDerived trait

[w0xlt]

There is currently no way to retrieve change addresses, so this PR changes the visibility of the derived module and the AsDerived trait.

With this change, the addresses can be retrieved using the function below, for example:

use bdk::descriptor::derived::AsDerived;

// ...

fn peek_change_address(&self, wallet: &Wallet<ElectrumBlockchain, MemoryDatabase>, index: u32) -> Result<AddressInfo, Error> {

        let result_descriptor = wallet.get_descriptor_for_keychain(bdk::KeychainKind::Internal);

        result_descriptor
            .as_derived(index, wallet.secp_ctx())
            .address(wallet.network())
            .map(|address| AddressInfo { index, address })
            .map_err(|_| Error::ScriptDoesntHaveAddressForm)
    }

[LLFourn]

I happened to need this feature for gun. Here's the (slightly messy) commit that adds the feature: LLFourn@e0950b9

I think this is a better way to do this. I'll see if I can clean up this commit and upstream it.

[LLFourn]

Here I've made a PR that allows you to do this: #522

[rajarshimaitra]

ACK, #522 is much cleaner way to do it.

[w0xlt]

Agree.
#522 is a better approach to achieve this change.
I will close this PR.