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 pull request updates the Kotlin project to build with Gradle 7.0. 
This is necessary to support building the project with Java 16.

This change adds a generic framework for modeling resource consumptions and
adapts opendc-simulator-compute to model machines and VMs on top of
this framework.

This framework anticipates the addition of additional resource types
such as memory, disk and network to the OpenDC codebase.

This change uses the Java Platform functionality from Gradle to enable
shared dependency constraints across modules.

This change updates the Gradle configuration to utilize version
constraints to force the same dependency version across modules.

This change moves the version of the dependencies from buildSrc to
gradle.properties to prevent recompilation when changing dependency
versions.

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