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/ExecutableJar.kt

@@ -0,0 +1,73 @@
+/*
+ * 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.file.RegularFileProperty
+import org.gradle.api.provider.ListProperty
+import org.gradle.api.tasks.Input
+import org.gradle.api.tasks.InputFile
+import org.gradle.api.tasks.OutputFile
+import org.gradle.api.tasks.TaskAction
+
+/**
+ * Builds a self-contained Pkl CLI Jar that is directly executable on Windows, macOS, and Linux.
+ *
+ * For direct execution, the `java` command must be on the PATH.
+ *
+ * Technique borrowed from [Mill](https://mill-build.org/blog/5-executable-jars.html).
+ */
+abstract class ExecutableJar : DefaultTask() {
+  @get:InputFile abstract val inJar: RegularFileProperty
+
+  @get:OutputFile abstract val outJar: RegularFileProperty
+
+  @get:Input abstract val jvmArgs: ListProperty<String>
+
+  @TaskAction
+  @Suppress("unused")
+  fun buildJar() {
+    val inFile = inJar.get().asFile
+    val outFile = outJar.get().asFile
+    val escapedJvmArgs = jvmArgs.get().joinToString(separator = " ") { "\"$it\"" }
+    val unixStartScript =
+      """
+      @ 2>/dev/null # 2>nul & echo off & goto BOF
+      :
+      exec java $escapedJvmArgs -jar "$0" "$@"
+      exit
+      """
+        .trimIndent()
+    val windowsStartScript =
+      """
+      :BOF
+      setlocal
+      @echo off
+      java $escapedJvmArgs -jar "%~dpnx0" %*
+      endlocal
+      exit /B %errorlevel%
+      """
+        .trimIndent()
+        // need crlf endings for Windows portion of script
+        .replace("\n", "\r\n")
+    val startScript = unixStartScript + "\r\n" + windowsStartScript + "\r\n".repeat(3)
+    outFile.outputStream().use { outStream ->
+      startScript.byteInputStream().use { it.copyTo(outStream) }
+      inFile.inputStream().use { it.copyTo(outStream) }
+    }
+
+    // chmod a+x
+    outFile.setExecutable(true, false)
+  }
+}