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/build.gradle.kts

@@ -0,0 +1,57 @@
+/*
+ * 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.
+ */
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
+plugins {
+  `kotlin-dsl`
+  `jvm-toolchains`
+}
+
+/**
+ * To avoid the provisioning of multiple JDKs and other build issues, keep this value in sync with
+ * the JVM toolchain versions in `BuildInfo.kt` and `gradle-daemon-jvm.properties`.
+ */
+val toolchainVersion = 25
+
+dependencies {
+  implementation(libs.downloadTaskPlugin)
+  implementation(libs.errorPronePlugin)
+  implementation(libs.nullawayPlugin)
+  implementation(libs.spotlessPlugin)
+  implementation(libs.kotlinPlugin) { exclude(module = "kotlin-android-extensions") }
+  implementation(libs.shadowPlugin)
+
+  // fix from the Gradle team: makes version catalog symbols available in build scripts
+  // see here for more: https://github.com/gradle/gradle/issues/15383
+  implementation(files(libs.javaClass.superclass.protectionDomain.codeSource.location))
+}
+
+java {
+  toolchain {
+    languageVersion = JavaLanguageVersion.of(toolchainVersion)
+    vendor = JvmVendorSpec.ADOPTIUM
+  }
+}
+
+tasks.withType<JavaCompile>().configureEach { options.release = toolchainVersion }
+
+kotlin {
+  jvmToolchain(toolchainVersion)
+  compilerOptions {
+    jvmTarget = JvmTarget.fromTarget(toolchainVersion.toString())
+    freeCompilerArgs.add("-Xjdk-release=$toolchainVersion")
+  }
+}