textproc/kepubify: update to 4.0.2

• Update to 4.0.2
• Convert to GO_MODULE

Upstream changes:

• Don't fail seriesmeta/covergen on scan errors in subdirectories
• Convert all input character encodings to UTF-8.

Changelog: https://github.com/pgaskin/kepubify/releases/tag/v4.0.2
PR: 260259