Empty env on init

[AlbertDeFusco]

With this PR anaconda-project init will write an empty packages: key to the anaconda-project.yml file.

Since Conda version 4.6 empty environments have been supported.

>anaconda-project init
Project configuration is in /Users/adefusco/Development/AnacondaPlatform/anaconda-project/examples/init-me/anaconda-project.yml
>anaconda-project list-packages
Conda packages for environment 'default':

>anaconda-project prepare
Collecting package metadata (current_repodata.json): ...working... done
Solving environment: ...working... done

## Package Plan ##

  environment location: /Users/adefusco/Development/AnacondaPlatform/anaconda-project/examples/init-me/envs/default

Preparing transaction: ...working... done
Verifying transaction: ...working... done
Executing transaction: ...working... done
#
# To activate this environment, use
#
#     $ conda activate /Users/adefusco/Development/AnacondaPlatform/anaconda-project/examples/init-me/envs/default
#
# To deactivate an active environment, use
#
#     $ conda deactivate

The project is ready to run commands.
Use `anaconda-project list-commands` to see what's available.

>anaconda-project add-packages python=3.8 flask
Collecting package metadata (current_repodata.json): ...working... done
Solving environment: ...working... done

## Package Plan ##

  environment location: /Users/adefusco/Development/AnacondaPlatform/anaconda-project/examples/init-me/envs/default

  added / updated specs:
    - flask
    - python=3.8

...

Executing transaction: ...working... done
Using Conda environment /Users/adefusco/Development/AnacondaPlatform/anaconda-project/examples/init-me/envs/default.
Added packages to project file: python=3.8, flask.

[AlbertDeFusco]

closing in favor of #322