restructure the CLI: sub-commands

[jkowalleck]

current CLI is described here: https://github.com/CycloneDX/cyclonedx-python#basic-usage

proposal: change the CLI to use proper sub-commands - one for each ecosystem:

$ cyclonedx-py --help
$ cyclonedx-py conda --help
$ cyclonedx-py poetry --help
$ cyclonedx-py environment --help
$ cyclonedx-py pipenv --help
$ cyclonedx-py requirements --help

benefits:

• each sub command could describe its own CLI - own options, own switches, own arguments - and own help pages of cause.
  for example cyclonedx requirements requires an argument to the requirement-file - which defaults to requirements.txt,
  while cyclonedx-py environment does not.
• ... to be continued ...

---

and if each of the sub commands would act autonomous, it could even become available as a public python module:

$ python -m cyclonedx_py.cli --help
$ python -m cyclonedx_py.cli.conda --help
# ... and so on 

[jkowalleck]

done