This change updates the project structure to become flattened.
Previously, the simulator, frontend and API each lived into their own
directory.

With this change, all modules of the project live in the top-level
directory of the repository. This should improve discoverability of
modules of the project.

This change removes the opendc-core module. This module was an artifact
of the old codebase and remained mostly unused. This change removes all
usages of the module and if necessary introduces replacement classes.

This change extracts the API for the OpenDC Compute service into a separate
module to establish a clearer boundary between the interface meant for
consumers and interfaces meant for the the serve implementation.