Move interface -> interfaces

Summary:
Core PR 12906 ( https://github.com/bitcoin/bitcoin/pull/12906/files ) renames these files.
Rather than applying this large refactor patch after the completion of T417, I'm applying part of it now
and will be applying the rest of it in-place for the remaining T417 diffs.

Test Plan:
make check
ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2826