From https://www.avidemux.org/admWiki/doku.php?id=build:install_2.6 : "Everything is a module in 2.6, so without plugins you can't do anything. The plugins either belong to the common part (they work with all UIs) or a specific part (they only work with one)." Reproducible: Didn't try
The repository is the same for software, core and plugins (and the same source tarball for each of them), there's no sense to separate them. Only localization modules are in another repository. Well, media-video/avidemux media-libs/avidemux-core media-libs/avidemux-plugins should be merged.