remove support for deprecated hydra override syntax

[Jasha10]

This PR removes deprecated support for overriding hydra config groups without use of the keyword override (see this deprecation notice).

Removing this deprecated behavior enables the use of custom subgroups of the top-level hydra config.

Example:

Suppose we have a directory tree like this:

├── conf
│   └── hydra
│       └── run
│           └── my_run.yaml
└── my_app.py

Let's say my_app.py is a minimal Hydra app:

# my_app.py
import hydra

@hydra.main(config_path="conf", config_name=None)
def app(cfg):
    print(cfg)

if __name__ == "__main__":
    app()

Before this PR, it is impossible to add hydra/run: my_run to the defaults list (since hydra/run does not appear in the defaults list of HydraConf).

$ # Before this PR:
$ python my_app.py hydra.job.chdir=true +hydra/run=my_run
/Users/jasha10/hydra.git/hydra/_internal/defaults_list.py:413: UserWarning: In _dummy_empty_config_: Invalid overriding of hydra/run:
Default list overrides requires 'override' keyword.
See https://hydra.cc/docs/next/upgrades/1.0_to_1.1/defaults_list_override for more information.

  deprecation_warning(msg)
In '_dummy_empty_config_': Could not override 'hydra/run'. No match in the defaults list.

Set the environment variable HYDRA_FULL_ERROR=1 for a complete stack trace.

$ # After this PR:
$ python my_app.py hydra.job.chdir=true +hydra/run=my_run
{}

This enabled use-case is relevant to discussion #2051.

[lgtm-com]

This pull request introduces 1 alert when merging 8dfea9d into c707ac2 - view on LGTM.com

new alerts:

• 1 for Unused import

[jieru-hu]

thanks @Jasha10
what would users see if they still have the old style override (- hydra/job_logging: colorlog) in their defaults list?

[Jasha10]

Using the old style override:

defaults:
  - hydra/job_logging: colorlog
  - hydra/hydra_logging: colorlog

$ python my_app.py hydra.job.chdir=true
Multiple values for hydra/job_logging. To override a value use 'override hydra/job_logging: colorlog'

Set the environment variable HYDRA_FULL_ERROR=1 for a complete stack trace.

[Jasha10]

I'm marking this PR as a draft until the new Hydra base_version functionality is in place.