Fix CLI runner

[dchandan]

Something is not working as expected with the CLI runner that @fmigneault put in place. The help text produced by running stac-populator should give a list of populations available, but right now it doesn't do that:

usage: stac-populator [-h] [--version] [--debug] {run} ...

STACpopulator operations.

options:
  -h, --help     show this help message and exit
  --version, -V  prints the version of the library and exits
  --debug        Set logger level to debug

command:
  STAC populator command to execute.

  {run}
    run          STACpopulator implementation runner.

[fmigneault]

You have to call stac-populator run --help to list implementations.

stac-populator --help is to list available commands. For now, only run is defined, but we could have others to interact with STAC in different manners.

[dchandan]

Oh right! Silly me.