curtin package fails to import curtin module

On a wily VM, installing curtin from archive doesn't produce a working curtin:

1. uvt-kvm create release=wily
2. sudo apt-get install curtin
3. curtin --help

ubuntu@w2:~$ /usr/bin/curtin --help
/usr/bin/python3: Error while finding spec for 'curtin.deps.install' (<class 'ImportError'>: No module named 'curtin')
failed to install deps!

curtin feels we should be running python3 (which we should, it's available on wily) but the deb package doesn't depend on python3-curtin, which is needed to resolve this import error.

After I run: $ sudo apt-get install python3-curtin
then it works:

$ /usr/bin/curtin --help
usage: main.py [-h] [--showtrace] [-v] [--log-file LOG_FILE] [-c FILE]
               [--set key=val]
               {apply_net,block-meta,curthooks,extract,hook,in-target,install,mkfs,net-meta,pack,swap,system-upgrade}
               ...

positional arguments:
  {apply_net,block-meta,curthooks,extract,hook,in-target,install,mkfs,net-meta,pack,swap,system-upgrade}

optional arguments:
  -h, --help show this help message and exit
  --showtrace
  -v, --verbose
  --log-file LOG_FILE
  -c FILE, --config FILE
                        read configuration from cfg
  --set key=val define a config variable. key can be a "/" delimited
                        path ("early_commands/cmd1=a"). if key starts with
                        "json:" then val is loaded as json
                        (json:stages="['early']")