Point the missing-module error and README at pip install

[mikelangmayr]

• camera-interface now ships a pyproject.toml, so camera_interface installs with pip install <camera-interface> instead of being built and found via PYTHONPATH
• Update ModuleNotAvailable to name that command, so the error says what to run
• Rewrite the README prerequisite for the same flow, and note that the instrument is fixed when the module is built
• Update the test that asserted the old message to assert the new one
• 14 tests pass; pylint reports nothing new (the existing missing-docstring messages in the tests predate this)