Also improve wording for Debian packages. Fixes #5940.
Part of #5613.
This change adds a virtual package `keyman-engine` which ibus-keyman provides. keyman-config can then depend on this virtual package. This allows to use fcitx instead of ibus if the fcitx-keyman package provides that virtual package as well.
This change consolidates the Debian packages into a single source package, with the exception of the legacy KMFL packages. Fixes #5022.