Autogenerate default env

[AlbertDeFusco]

This allows the simplest anaconda-project.yml file to be utilized without an env_specs key.

name: My project

packages:
  - python=3.7
  - notebook
channels: []

on anaconda-project prepare it will infer that the name of the env_spec is default.

[AlbertDeFusco]

This also works on environment.yml-like files. This PR could be extended to just assume the environment.yml file is the anaconda-project.yml. Anyone have strong opinions here?

name: no_envs
dependencies:
  - python=3.7
  - requests

[jbednar]

I'm not sure of the precise thing you are proposing, but it sounds in line with my own opinion that there shouldn't be any major distinctions between an environment.yml file and an anaconda-project.yml file; as far as environments are concerned they should be made to be interchangeable.

[mcg1969]

I am all for this. I would definitely support interpreting environment.yml files this way if there is no anaconda-project.yml file in the same directory.

[mcg1969]

@AlbertDeFusco this is passing. We can get this in now if you like, and handle the environment.yml case separately.

[AlbertDeFusco]

certainly