All tasks files are moved to ./tasks/<codename> subdirectories so they
can be more easily kept up-to-date with specific distro releases.
Always generate the debian-installer and tasksel tasks dynamically:
- all task files for the desired suite are copied to the working
directory at the beginning of each build, and are used from there;
- the debian-installer and tasksel tasks are no longer included in
releases but are always created automatically at build time; this
means users no longer need to run the generate_di_* scripts or the
update_tasksel script;
- the popcon task file will remain included as a static file, but plan
is to add an option to update it automatically for each build; reason
is that updating it requires network access.
Bump version to 3.1.