[wallet] Refactor ProcessImport() to call ProcessImportLegacy()

Summary:
This is almost entirely a move-only commit.

Reviewer hint: use --color-moved=zebra for review.

This is a partial backport of Core PR14491 : https://github.com/bitcoin/bitcoin/pull/14491/commits/d2b381cc91b2c4e74abe11e5bd66af647b70dafb

Depends on D6189

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6190