Have importprivkey use CWallet's ImportPrivKeys, ImportScripts, and ImportScriptPubKeys

Summary:
Behavior changes:

• If we already have the key, it's wpkh script will still be added, although it should already be there

Tis is a partial backport of Core PR16301 : https://github.com/bitcoin/bitcoin/pull/16301/commits/c6a827424711333f6f66cf5f9d79e0e6884769de

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6412