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 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.

This change removes unnecessary dependencies on JUnit Platform launcher
from the repository. Previously, the launcher was used to bootstrap
tests for Gradle when it did not natively support JUnit Platform.

Gradle now has native support for JUnit Platform, so the dependency is
not needed anymore.

This change adds explicit visibility modifiers to the public interfaces
and enables Kotlin 1.4's explicit API mode.

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.