Move locale migration after cleanup

[chrisawi]

Otherwise, module-level cleanup can remove locale symlinks that point to locale data installed by other modules.

Closes #154

I've tested this building an app, and for that at least, it seems to work. I haven't tried building a runtime though. There are two other calls to builder_migrate_locale_dirs() that I haven't really investigated. One happens in init for all runtimes, and the other applies only to platforms.

[TingPing]

If you could, add some tests to https://github.com/flatpak/flatpak-builder/blob/main/tests/test-builder.sh