Missing dot at the end of docstring
Cleanup related to the use of the name "environment" and "env"

Before :
- There was an "environment" module, with an "Environment" class
- by convention, "Environment" instances were named "env"
- but utils.execute takes an "env" argument which is not an Environment
instance but a dict mapping of environment variables ; That's
inconsistent !

Now :
- Renamed the "environment" module to "environments" ; that's more
coherent with other modules like "packages", "repositories", etc.
- Renamed the Environment instances "environment" ; that's the new
convention
- From now, environment variables dict mapping will be named "env"