Allow wallet files not in -walletdir directory

Summary:

Remove restriction that -wallet filenames can only refer to files in the
-walletdir directory.

Partial backport of core PR11687 (commit 26c06f2)
https://github.com/bitcoin/bitcoin/pull/11687/commits/26c06f24e5dcc32a7599abb8d670d22993c82bc2

Depends on D3103
Progress towards T651

Test Plan:

make check
./test/functional/test_runner.py wallet_multiwallet

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3104