Move interface -> interfaces
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.
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: deadalnix, Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D2826