[Bug] Appending to Defaults List via command line doesn't work if config is in subfolder

[siferati]

🐛 Bug

Description

When overriding config groups in the terminal, hydra interprets them as being in the global package.
This has been stated in different issues:

• [Question] Override a config group of absolute path from command line #2434 (comment)
• [Bug] Absolute path for overrides group does not work like defaults list #2105 (comment)

However, this doesn't seem to work when appending to configs loaded via --config-name=subfolder/config_name.
Hydra takes the given override as a relative path and fails to append it (see below for minimal example)

Checklist

• I checked on the latest version of Hydra
• I created a minimal repro (See this for tips).

To reproduce

# see vanilla config
> python main.py --config-name=server/alpha
db:
  driver: mysql
  user: omry
  password: secret
name: alpha

# override mysql db with postgresql -- works as expected
> python main.py --config-name=server/alpha db=postgresql
db:
  driver: postgresql
  user: postgres_user
  password: drowssap
  timeout: 10
name: alpha

# append a new db configuration -- doesn't work because unlike override, append is treating postgresql as relative path
> python main.py --config-name=server/alpha +db@db_2=postgresql
hydra.errors.MissingConfigException: In 'server/alpha': Could not find 'server/db/postgresql'

Stack trace/error message:

Traceback (most recent call last):
  File "test/main.py", line 11, in <module>
    my_app()
  File "/virtual_env/lib/python3.10/site-packages/hydra/main.py", line 90, in decorated_main
    _run_hydra(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/utils.py", line 394, in _run_hydra
    _run_app(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/utils.py", line 457, in _run_app
    run_and_report(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/utils.py", line 222, in run_and_report
    raise ex
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/utils.py", line 219, in run_and_report
    return func()
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/utils.py", line 458, in <lambda>
    lambda: hydra.run(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/hydra.py", line 105, in run
    cfg = self.compose_config(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/hydra.py", line 594, in compose_config
    cfg = self.config_loader.load_configuration(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 142, in load_configuration
    return self._load_configuration_impl(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 253, in _load_configuration_impl
    defaults_list = create_defaults_list(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 746, in create_defaults_list
    defaults, tree = _create_defaults_list(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 716, in _create_defaults_list
    defaults_tree = _create_defaults_tree(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 356, in _create_defaults_tree
    ret = _create_defaults_tree_impl(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 457, in _create_defaults_tree_impl
    return _expand_virtual_root(repo, root, overrides, skip_missing)
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 280, in _expand_virtual_root
    subtree = _create_defaults_tree_impl(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 573, in _create_defaults_tree_impl
    add_child(children, new_root)
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 520, in add_child
    subtree_ = _create_defaults_tree_impl(
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 488, in _create_defaults_tree_impl
    config_not_found_error(repo=repo, tree=root)
  File "/virtual_env/lib/python3.10/site-packages/hydra/_internal/defaults_list.py", line 800, in config_not_found_error
    raise MissingConfigException(
hydra.errors.MissingConfigException: In 'server/alpha': Could not find 'server/db/postgresql'

Expected Behavior

Appending treats the given config as a global package, just like when overriding.
The following command should work instead of resulting in an error:

> python main.py --config-name=server/alpha +db@db_2=postgresql
db:
  driver: mysql
  user: omry
  password: secret
name: alpha
db_2:
  driver: postgresql
  user: postgres_user
  password: drowssap
  timeout: 10

System information

• Hydra Version : 1.3.0 (tested 1.3.2 too, same problem)
• Python version : 3.10.12
• Virtual environment type and version : virtualenv 20.16.5
• Operating system : Ubuntu 22.04