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/CCTweakedExtension.kt

@@ -5,6 +5,7 @@ import org.gradle.api.Project
 import org.gradle.api.attributes.TestSuiteType
 import org.gradle.api.file.FileSystemOperations
 import org.gradle.api.provider.Provider
+import org.gradle.api.provider.SetProperty
 import org.gradle.api.reporting.ReportingExtension
 import org.gradle.api.tasks.JavaExec
 import org.gradle.api.tasks.SourceSetContainer
@@ -31,7 +32,8 @@ abstract class CCTweakedExtension(
 
     /** Get the current git branch. */
     val gitBranch: Provider<String> = gitProvider(project, "<no git branch>") {
-        ProcessHelpers.captureOut("git", "-C", project.projectDir.absolutePath, "rev-parse", "--abbrev-ref", "HEAD").trim()
+        ProcessHelpers.captureOut("git", "-C", project.projectDir.absolutePath, "rev-parse", "--abbrev-ref", "HEAD")
+            .trim()
     }
 
     /** Get a list of all contributors to the project. */
@@ -64,6 +66,30 @@ abstract class CCTweakedExtension(
         contributors.sortedWith(String.CASE_INSENSITIVE_ORDER)
     }
 
+    /**
+     * References to other sources
+     */
+    val sourceDirectories: SetProperty<SourceSetReference> =
+        project.objects.setProperty(SourceSetReference::class.java)
+
+    /** All source sets referenced by this project. */
+    val sourceSets = sourceDirectories.map { x -> x.map { it.sourceSet } }
+
+    init {
+        sourceDirectories.finalizeValueOnRead()
+    }
+
+    /**
+     * Mark this project as consuming another project. Its [sourceDirectories] are added, ensuring tasks are set up
+     * correctly.
+     */
+    fun externalSources(project: Project) {
+        val otherCct = project.extensions.getByType(CCTweakedExtension::class.java)
+        for (sourceSet in otherCct.sourceSets.get()) {
+            sourceDirectories.add(SourceSetReference.external(sourceSet))
+        }
+    }
+
     fun jacoco(task: NamedDomainObjectProvider<JavaExec>) {
         val classDump = project.buildDir.resolve("jacocoClassDump/${task.name}")
         val reportTaskName = "jacoco${task.name.capitalized()}Report"

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

@@ -2,14 +2,28 @@ package cc.tweaked.gradle
 
 import org.gradle.api.Plugin
 import org.gradle.api.Project
+import org.gradle.api.tasks.SourceSetContainer
 import org.gradle.jvm.toolchain.JavaLanguageVersion
+import org.gradle.testing.jacoco.tasks.JacocoReport
 
 /**
  * Configures projects to match a shared configuration.
  */
 class CCTweakedPlugin : Plugin<Project> {
     override fun apply(project: Project) {
-        project.extensions.create("cct", CCTweakedExtension::class.java)
+        val sourceSets = project.extensions.getByType(SourceSetContainer::class.java)
+
+        val cct = project.extensions.create("cct", CCTweakedExtension::class.java)
+        cct.sourceDirectories.add(SourceSetReference.internal(sourceSets.getByName("main")))
+
+        project.afterEvaluate {
+            cct.sourceDirectories.disallowChanges()
+        }
+
+        // Set up jacoco to read from /all/ our source directories.
+        project.tasks.named("jacocoTestReport", JacocoReport::class.java) {
+            for (ref in cct.sourceSets.get()) sourceDirectories.from(ref.allSource.sourceDirectories)
+        }
     }
 
     companion object {

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)
+    }
+}