K2: Allow for private dependencies automatically installed with make venv but not part of repo

[per-bohlin]

Some users run K2 from within the repository. To make it simpler to maintain private addons and additional 3rd party dependencies, create a mechanism where private.requirements.txt are automatically installed in the venv when running make venv.
The private.requirements.txt file should be added to the .gitignore list.
The make venv target must be able to handle that the file private.requirements.txt is not present.