Gradle: Replace legacy buildSrc mechanism with included build (#1524)

Motivation:
buildSrc is a special-case legacy mechanism.
Gradle recommends using an included build named build-logic instead:

https://docs.gradle.org/current/userguide/best_practices_structuring_builds.html#favor_composite_builds

Changes:
- Rename buildSrc/ to build-logic/
  - triggers reformatting
- Replace occurrences of "buildSrc" with "build-logic"
- Include the build-logic build in the main build (via
settings.gradle.kts)
- Apply convention plugins via plugin IDs instead of type-safe accessors
  - small tradeoff compared to buildSrc

Result:
- Faster and more isolated builds
- Build logic behaves like a normal build, making it easier to evolve
and reason about

---------

Co-authored-by: Daniel Chao <dan.chao@apple.com>

build-logic/src/main/kotlin/PklFormatterSpotless.kt

@@ -0,0 +1,58 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import com.diffplug.spotless.FormatterFunc
+import com.diffplug.spotless.FormatterStep
+import java.io.Serial
+import java.io.Serializable
+import java.net.URLClassLoader
+import org.gradle.api.artifacts.Configuration
+
+class PklFormatterStep(@Transient private val configuration: Configuration) : Serializable {
+  companion object {
+    @Serial private const val serialVersionUID: Long = 1L
+  }
+
+  fun create(): FormatterStep {
+    return FormatterStep.createLazy(
+      "pkl",
+      { PklFormatterStep(configuration) },
+      { PklFormatterFunc(configuration) },
+    )
+  }
+}
+
+class PklFormatterFunc(@Transient private val configuration: Configuration) :
+  FormatterFunc, Serializable {
+  companion object {
+    @Serial private const val serialVersionUID: Long = 1L
+  }
+
+  private val classLoader by lazy {
+    val urls = configuration.files.map { it.toURI().toURL() }
+    // Use the platform classloader as parent to isolate from Gradle's classloader
+    URLClassLoader(urls.toTypedArray(), ClassLoader.getPlatformClassLoader())
+  }
+
+  private val formatterClass by lazy { classLoader.loadClass("org.pkl.formatter.Formatter") }
+
+  private val formatMethod by lazy { formatterClass.getMethod("format", String::class.java) }
+
+  private val formatterInstance by lazy { formatterClass.getConstructor().newInstance() }
+
+  override fun apply(input: String): String {
+    return formatMethod(formatterInstance, input) as String
+  }
+}