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/pklJavaLibrary.gradle.kts

@@ -0,0 +1,123 @@
+/*
+ * Copyright © 2024-2026 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.
+ */
+@file:Suppress("HttpUrlsUsage", "unused")
+
+import org.gradle.accessors.dm.LibrariesForLibs
+
+plugins {
+  `java-library`
+  `jvm-toolchains`
+  `jvm-test-suite`
+  id("pklKotlinTest")
+  id("com.diffplug.spotless")
+}
+
+// make sources Jar available to other subprojects
+val sourcesJarConfiguration: Provider<Configuration> = configurations.register("sourcesJar")
+
+// Version Catalog library symbols.
+val libs = the<LibrariesForLibs>()
+
+// Build configuration.
+val info = project.extensions.getByType<BuildInfo>()
+
+java {
+  withSourcesJar() // creates `sourcesJar` task
+  withJavadocJar()
+
+  toolchain {
+    languageVersion = info.jdkToolchainVersion
+    vendor = info.jdkVendor
+  }
+}
+
+artifacts {
+  // make sources Jar available to other subprojects
+  add("sourcesJar", tasks["sourcesJar"])
+}
+
+spotless {
+  val revertYearOnlyChanges = RevertYearOnlyChangesStep(rootProject.rootDir, ratchetFrom!!).create()
+
+  java {
+    addStep(revertYearOnlyChanges)
+    googleJavaFormat(libs.versions.googleJavaFormat.get())
+    target("src/*/java/**/*.java")
+    licenseHeaderFile(
+      rootProject.file("build-logic/src/main/resources/license-header.star-block.txt")
+    )
+  }
+}
+
+tasks.jar {
+  manifest {
+    attributes +=
+      mapOf(
+        "Automatic-Module-Name" to "org.${project.name.replace("-", ".")}",
+        "Add-Exports" to info.jpmsExportsForJarManifest,
+      )
+  }
+}
+
+tasks.javadoc {
+  classpath = sourceSets.main.get().output + sourceSets.main.get().compileClasspath
+  source = sourceSets.main.get().allJava
+  title = "${project.name} ${project.version} API"
+  (options as StandardJavadocDocletOptions).addStringOption("Xdoclint:none", "-quiet")
+}
+
+val truffleJavacArgs =
+  listOf(
+    // TODO: determine correct limits for Truffle specializations
+    // (see https://graalvm.slack.com/archives/CNQSB2DHD/p1712380902746829)
+    "-Atruffle.dsl.SuppressWarnings=truffle-limit"
+  )
+
+tasks.compileJava {
+  javaCompiler = info.javaCompiler
+  options.compilerArgs.addAll(truffleJavacArgs + info.jpmsAddModulesFlags)
+}
+
+tasks.withType<JavaCompile>().configureEach {
+  javaCompiler = info.javaCompiler
+  options.release = info.jvmTarget
+}
+
+tasks.withType<JavaExec>().configureEach { jvmArgs(info.jpmsAddModulesFlags) }
+
+fun Test.configureJdkTestTask(launcher: Provider<JavaLauncher>) {
+  useJUnitPlatform()
+  javaLauncher = launcher
+  systemProperties.putAll(info.testProperties)
+  jvmArgs.addAll(info.jpmsAddModulesFlags)
+}
+
+tasks.test { configureJdkTestTask(info.javaTestLauncher) }
+
+// Prepare test tasks for each JDK version which is within the test target suite for Pkl. Each task
+// uses a pinned JDK toolchain version, and is named for the major version which is tested.
+//
+// Test tasks configured in this manner are executed manually by name, e.g. `./gradlew testJdk11`,
+// and automatically as dependencies of `check`.
+//
+// We omit the current JDK from this list because it is already tested, in effect, by the default
+// `test` task.
+//
+// Pkl subprojects may elect to further configure these tasks as needed; by default, each task
+// inherits the configuration of the default `test` task (aside from an overridden launcher).
+val jdkTestTasks = info.multiJdkTestingWith(tasks.test) { (_, jdk) -> configureJdkTestTask(jdk) }
+
+tasks.check { dependsOn(jdkTestTasks) }