Move the core API into a separate module

It should be possible to consume the ComputerCraft's core (i.e.
non-Minecraft code) in other projects, such as emulators.  While this
has been possible for years, it's somewhat tricky from a maintenance
perspective - it's very easy to accidentally add an MC dependency
somewhere!

By publishing a separate "core" jar, we can better distinguish the
boundaries between our Lua runtime and the Minecraft-specific code.

Ideally we could have one core project (rather than separate core and
core-api modules), and publish a separate "api" jar, like we do for the
main mod. However, this isn't really possible to express using Maven
dependencies, and so we must resort to this system.

Of course, this is kinda what the Java module system is meant to solve,
but unfortunately getting that working with Minecraft is infeasible.

buildSrc/src/main/kotlin/cc/tweaked/gradle/SourceSetReference.kt

@@ -0,0 +1,20 @@
+package cc.tweaked.gradle
+
+import org.gradle.api.tasks.SourceSet
+
+data class SourceSetReference(
+    val sourceSet: SourceSet,
+    val classes: Boolean,
+    val external: Boolean,
+) {
+    companion object {
+        /** A source set in the current project. */
+        fun internal(sourceSet: SourceSet) = SourceSetReference(sourceSet, classes = true, external = false)
+
+        /** A source set from another project. */
+        fun external(sourceSet: SourceSet) = SourceSetReference(sourceSet, classes = true, external = true)
+
+        /** A source set which is inlined into the current project. */
+        fun inline(sourceSet: SourceSet) = SourceSetReference(sourceSet, classes = false, external = false)
+    }
+}