Directory structure - Split `spacetime` into multiple binaries

[bfops]

Background

We would like to split spacetime into spacetimedb-cli and spacetimedb-standalone.

Definition of "Done"

TODO

[gefjon]

Noa: there will be spacetime in /usr/bin, symlinked to spacetime-update, which is maybe not versioned, or versioned separately from the CLI? When spacetime-update is invoked with argv[0] == spacetime, it will exec the spacetime-cli binary. spacetime-update functionality accessible as the spacetime version subcommand, like spacetime version install &c.