Move some more build logic to buildSrc

Look, I don't enjoy having 600 LOC long build.gradle files, it's just
very easy to do! This at least moves some of the complexity elsewhere,
so the build script is a little more declarative.

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

@@ -0,0 +1,13 @@
+package cc.tweaked.gradle
+
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+
+/**
+ * Configures projects to match a shared configuration.
+ */
+class CCTweakedPlugin : Plugin<Project> {
+    override fun apply(project: Project) {
+        project.extensions.create("cct", CCTweakedExtension::class.java)
+    }
+}