* guix-jupyter-kernel.scm (session-id): Remove.
(create-environment): Call 'spawn-kernel/container' instead of
'start-container', and remove 'name' argument.
(%network-file-systems, module-to-import?, start-container): Move to...
* guix/jupyter/kernel.scm: ... here.
(start-container): Rename to...
(spawn-kernel/container): ... this.  Remove 'name' parameter.  Call
'make-container-root-directory' to create the root directory of the
container.
(make-container-root-directory): New procedure.