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 adds support for aggregating code coverage results from the
different modules.

This change extracts the configuration for test from the Kotlin library
conventions.

This change moves the OpenDC simulator codebase to the org.opendc
namespace of which we control the domain. Previously, we used the
com.atlarge package of which we did not control the domain, which might
lead to difficulties in the future.

This change moves the OpenDC modules previously living in the simulator/opendc
directory to the simulator directory itself given that we do not make a
distinction between OpenDC and odcsim anymore.

This commit updates the Gradle wrapper to version 6.5.1 to address some
of the issues we were having when importing the project.

This change prepares the simulator Git repository for the monorepo residing at
https://github.com/atlarge-research.com/opendc. To accomodate for this, we
move all files into a simulator subdirectory.