Merge #14350: Add WalletLocation class

Summary:
65f3672f3b82a6fa30e5171f85bc8d8a29e0797e wallet: Refactor to use WalletLocation (João Barbosa)
01a4c095c87500650663341533f000c6b613e9da wallet: Add WalletLocation utility class (João Barbosa)

Pull request description:

Advantages of this change:
 - avoid resolving wallet absolute path and name repetitively and in multiple places;
 - avoid calling `GetWalletDir` in multiple places;
 - extract these details from the actual wallet implementation.

The `WalletLocation` class can be a way to represent a wallet not yet loaded that exists in the wallet directory.

Tree-SHA512: 71ec09786e038499710e7acafe92d66ab9883fc894964e267443ae9c10a6872a10995c3987a169c436a4e793dae96b28fb97bd7f78483c4b72ac930fa23f8686

Backport of Core PR14350
https://github.com/bitcoin/bitcoin/pull/14350/

Completes T596

Test Plan:

make check
test_runner.py wallet_*

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4739