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/ExecutableSpec.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright © 2025-2026 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.provider.Property
+
+abstract class ExecutableSpec {
+  /** The main entrypoint Java class of the executable. */
+  abstract val mainClass: Property<String>
+
+  /**
+   * The name of the native executable.
+   *
+   * Not required if not building a native executable.
+   */
+  abstract val name: Property<String>
+
+  /** The name of the Java executable. */
+  abstract val javaName: Property<String>
+
+  /** The name of the executable that shows in the description when published to Maven. */
+  abstract val documentationName: Property<String>
+
+  /**
+   * The base name of the Maven publication.
+   *
+   * This becomes the base name of the Artifact ID, with the os and arch suffixed.
+   *
+   * For example, `pkl` becomes `pkl-macos-aarch` for the macOS/aarch64 variant.
+   */
+  abstract val publicationName: Property<String>
+
+  /** The name of the artifact ID for the Java executable. */
+  abstract val javaPublicationName: Property<String>
+
+  /** The website for this executable. */
+  abstract val website: Property<String>
+}