Add commands to list inventory packages and groups

[jakduch]

Add two discoverable CLI actions backed by Ansible's inventory parser:

• obal list-packages prints the hosts in the packages group.
• obal list-groups prints user-defined inventory groups while omitting Ansible's implicit all and ungrouped groups.

Both commands are included in top-level help, produce sorted one-name-per-line output, honor OBAL_INVENTORY, and report the existing missing-inventory error consistently.

Tested against the upstream and Copr manifest fixtures; 79 focused unit and playbook/help tests pass, together with yamllint, Python compilation, and git diff --check.

Fixes #426
Fixes #427

[jakduch]

Fixed the CI failures: the two discovery playbooks now pass Ansible syntax checks, and the help fixtures use the Python 3.9 baseline that the existing compatibility normalization expects. The 79 focused inventory and playbook/help tests pass locally, together with both playbook syntax checks, yamllint and Pylint.

[jakduch]

Rebased onto the latest master and resolved the tests/test_unit.py conflict by keeping both the new rpmspec-query coverage from the base branch and the inventory-listing tests from this PR. The focused inventory and playbook/help suite still passes locally (79 tests). New head: 386bfce.

[jakduch]

The rebase is clean now, and the full CI run passed: https://github.com/theforeman/obal/actions/runs/36270848155