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/ResolveSourcesJars.kt

@@ -0,0 +1,60 @@
+/*
+ * 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.gradle.api.DefaultTask
+import org.gradle.api.artifacts.Configuration
+import org.gradle.api.artifacts.result.ResolvedArtifactResult
+import org.gradle.api.artifacts.result.ResolvedDependencyResult
+import org.gradle.api.file.DirectoryProperty
+import org.gradle.api.provider.Property
+import org.gradle.api.tasks.InputFiles
+import org.gradle.api.tasks.OutputDirectory
+import org.gradle.api.tasks.TaskAction
+import org.gradle.jvm.JvmLibrary
+import org.gradle.kotlin.dsl.property
+import org.gradle.language.base.artifact.SourcesArtifact
+
+open class ResolveSourcesJars : DefaultTask() {
+  @get:InputFiles val configuration: Property<Configuration> = project.objects.property()
+
+  @get:OutputDirectory val outputDir: DirectoryProperty = project.objects.directoryProperty()
+
+  @TaskAction
+  @Suppress("UnstableApiUsage", "unused")
+  fun resolve() {
+    val componentIds =
+      configuration.get().incoming.resolutionResult.allDependencies.map {
+        (it as ResolvedDependencyResult).selected.id
+      }
+
+    val resolutionResult =
+      project.dependencies
+        .createArtifactResolutionQuery()
+        .forComponents(componentIds)
+        .withArtifacts(JvmLibrary::class.java, SourcesArtifact::class.java)
+        .execute()
+
+    val resolvedJars =
+      resolutionResult.resolvedComponents
+        .flatMap { it.getArtifacts(SourcesArtifact::class.java) }
+        .map { (it as ResolvedArtifactResult).file }
+
+    // copying to an output dir because I don't know how else to describe task outputs
+    project.sync {
+      from(resolvedJars)
+      into(outputDir)
+    }
+  }
+}