Move cdds.common.plugins into cdds.plugins

Considering the size and importance of `cdds.common.plugins` it should probably moved to the root of the `cdds` package. This will be a breaking change for external plugins as imports will have to be updated, but if we end up refactoring/rewriting some of that code we might as well do this at the same time.

Start by locating the cdds.common.plugins package and the target cdds.plugins package, then search the repository for imports and references to cdds.common.plugins. Review how external plugins are expected to import it and update the relevant references as part of the move. Done means the package is available under cdds.plugins and the affected imports remain consistent.

Written by the indexing model from the issue text.