Have importwallet use ImportPrivKeys and ImportScripts

Summary:
Behavior changes:

• An "Importing ..." line is logged for every key, even ones that are skipped

This is a partial backport of Core PR16301 : https://github.com/bitcoin/bitcoin/pull/16301/commits/40ad2f6a58228c72c655e3061a19a63640419378

Depends on D6415

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6416