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/JavaVersionRange.kt

@@ -0,0 +1,65 @@
+/*
+ * 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("MemberVisibilityCanBePrivate")
+
+import java.util.*
+import org.gradle.jvm.toolchain.JavaLanguageVersion
+
+typealias JavaVersionPair = Pair<JavaLanguageVersion, JavaLanguageVersion>
+
+// All LTS releases.
+private val ltsReleases =
+  sortedSetOf(
+    JavaLanguageVersion.of(8),
+    JavaLanguageVersion.of(11),
+    JavaLanguageVersion.of(17),
+    JavaLanguageVersion.of(21),
+    JavaLanguageVersion.of(25),
+  )
+
+/** Describes an inclusive range of JVM versions, based on the [JavaLanguageVersion] type. */
+@JvmInline
+value class JavaVersionRange private constructor(private val bounds: JavaVersionPair) :
+  Iterable<JavaLanguageVersion> {
+  @Suppress("unused")
+  companion object {
+    fun isLTS(version: JavaLanguageVersion): Boolean = version in ltsReleases
+
+    fun inclusive(floor: JavaLanguageVersion, ceiling: JavaLanguageVersion): JavaVersionRange =
+      JavaVersionRange(floor to ceiling)
+
+    fun startingAt(floor: JavaLanguageVersion): JavaVersionRange =
+      inclusive(floor, JavaLanguageVersion.of(PKL_TEST_JDK_MAXIMUM))
+
+    fun upTo(ceiling: JavaLanguageVersion): JavaVersionRange =
+      inclusive(JavaLanguageVersion.of(PKL_TEST_JDK_MINIMUM), ceiling)
+  }
+
+  operator fun contains(version: JavaLanguageVersion): Boolean =
+    version >= bounds.first && version <= bounds.second
+
+  fun asSequence(): Sequence<JavaLanguageVersion> = sequence {
+    var current = bounds.first
+    while (current <= bounds.second) {
+      yield(current)
+      current = JavaLanguageVersion.of(current.asInt() + 1)
+    }
+  }
+
+  fun asSortedSet(): SortedSet<JavaLanguageVersion> = asSequence().toSortedSet()
+
+  override fun iterator(): Iterator<JavaLanguageVersion> = asSortedSet().iterator()
+}