Add support for running cleanup phase as part of export

[chergert]

Description of the problem

In Builder, we use the flatpak build-export ... feature to allow users to export their application with the local changes they've made in the IDE. However, this does not allow us to selectively run the "cleanup" phase where some features such as desktop file rename is done. Applications that use this (often development builds) are likely to be broken in this case.

We need either a way to selectively run the cleanup phase as part of build-export, or an additional command such as build-cleanup that we can manually run before build-export. When we perform this operation, Builder will know to cleanup any caches since incremental builds will have to be restarted from scratch.

[alexlarsson]

The cleanup phase depends on the cache state being correct though, so it can know which files were installed by each module so they can be removed. If the build is not clean that can go wrong.