fix CLI definition to find collection config file

[fmigneault]

Changes

• Replace logic to resolve and load specific implementation configuration file of a populator to avoid depending on
  inconsistent caller (python <impl-module.py> vs stac-populator run <impl>).
• Fix configuration file of populator implementation not found when package is installed.
• Allow a populator implementation to override the desired configuration file.
• Add missing CLI default="full" mode for CMIP6_UofT populator implementation.
• Fix Docker entrypoint to use stac-populator to make call to the CLI more convenient.
• Add get_logger function to avoid repeated configuration across modules.
• Make sure that each implementation and module employs their own logger.

[Nazim-crim]

lgtm!

[huard]

Gave a quick look, ok for me.