Support unpacked execution.

[jsirois]

Instead of a bare scie-jump executing a boot-pack of a lift manifest, it should gain the ability to also execute the lift manifest directly. The interface would be the same as the boot-pack in terms of an optional argument indicating the lift manifest to use, defaulting to the sibling lift.json. This would allow a scie to be tested and experimented with before final packaging as a monolithic scie binary. This notably would support Pants-style cache-efficient sandboxes while retaining the ability to run a final boot-pack in a packaging step.