Src packaging

[MattWellie]

• puts everything inside src
• we detect the multiple separate packages:
  • cpg_workflows for all the general pipeline code (and stuff that's not been split out yet)
  • talos for talos
  • clivarbitration for ...
• installation of this repository installs all relevant sub-packages
• running main.py no longer imports from the locally available files in Hail Batch

incomplete, just a food-for-thought proposal, twinned with #813