New `spack.environment.active_environment` api, and make spack.environment not depend on spack.cmd. (#25439)

* Refactor active environment getters

- Make `spack.environment.active_environment` a trivial getter for the active
environment, replacing `spack.environment.get_env` when the arguments are
not needed
- New method `spack.cmd.require_active_environment(cmd_name)` for 
commands that require an environment (rather than abusing 
get_env/active_environment)
- Clean up calling code to call spack.environment.active_environment or
spack.cmd.require_active_environment as appropriate
- Remove the `-e` parsing from `active_environment`, because `main.py` is
responsible for processing `-e` and already activates the environment.
- Move `spack.environment.find_environment` to
`spack.cmd.find_environment`, to avoid having spack.environment aware
of argparse.
- Refactor `spack install` command so argument parsing is all handled in the
command, no argparse in spack.environment or spack.installer
- Update documentation

* Python 2: toplevel import errors only with 'as ev'

In two files, `import spack.environment as ev` leads to errors
These errors are not well understood ("'module' object has no attribute
'environment'"). All other files standardize on the above syntax.

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