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/settings.gradle.kts

@@ -0,0 +1,39 @@
+/*
+ * 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("UnstableApiUsage")
+
+rootProject.name = "build-logic"
+
+pluginManagement {
+  repositories {
+    mavenCentral()
+    gradlePluginPortal()
+  }
+}
+
+plugins { id("org.gradle.toolchains.foojay-resolver-convention") version "1.0.0" }
+
+// makes ~/.gradle/init.gradle unnecessary and ~/.gradle/gradle.properties optional
+dependencyResolutionManagement {
+  // use same version catalog as main build
+  versionCatalogs { register("libs") { from(files("../gradle/libs.versions.toml")) } }
+
+  repositories {
+    repositoriesMode.set(RepositoriesMode.PREFER_SETTINGS)
+    mavenCentral()
+    gradlePluginPortal()
+  }
+}