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/pklJSpecify.gradle.kts

@@ -0,0 +1,46 @@
+/*
+ * 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 net.ltgt.gradle.errorprone.errorprone
+import net.ltgt.gradle.nullaway.nullaway
+import org.gradle.accessors.dm.LibrariesForLibs
+import org.gradle.api.tasks.compile.JavaCompile
+
+plugins {
+  `java-library`
+  id("net.ltgt.errorprone")
+  id("net.ltgt.nullaway")
+}
+
+val libs = the<LibrariesForLibs>()
+
+dependencies {
+  api(libs.jspecify)
+  errorprone(libs.errorProne)
+  errorprone(libs.nullaway)
+}
+
+nullaway { onlyNullMarked = true }
+
+tasks.withType<JavaCompile>().configureEach {
+  options.errorprone.disableAllChecks = true
+  options.errorprone.nullaway {
+    error()
+    onlyNullMarked = true
+    jspecifyMode = true
+    // honor assert x != null in addition to Objects.requireNonNull(x)
+    assertsEnabled = true
+  }
+}