Fix a dependency on kotlinx.serialization.core in JPMS for it to be truly optional (#507)

Do not mark the serialization JPMS dependency as transitive

See <https://stackoverflow.com/questions/77644309/are-jpms-modifiers-static-and-transitive-incompatible>

Additionally, move all integration tests into a separate directory.
There are too many of them at this point to keep them in the root.

Author: dkhalanskyjb

core/jvm/java9/module-info.java

@@ -1,6 +1,6 @@
 module kotlinx.datetime {
     requires transitive kotlin.stdlib;
-    requires transitive static kotlinx.serialization.core;
+    requires static kotlinx.serialization.core;
 
     exports kotlinx.datetime;
     exports kotlinx.datetime.serializers;

integration-testing/jpms-test/build.gradle.kts

@@ -0,0 +1,11 @@
+plugins {
+    kotlin("jvm")
+}
+
+java {
+    toolchain { languageVersion.set(JavaLanguageVersion.of(11)) }
+}
+
+dependencies {
+    api(project(":kotlinx-datetime"))
+}

integration-testing/jpms-test/src/main/java/module-info.java

@@ -0,0 +1,4 @@
+module my.jpms.test {
+    requires transitive kotlin.stdlib;
+    requires transitive kotlinx.datetime;
+}

integration-testing/jpms-test/src/main/java/my/jpms/test/InitLocalDate.java

@@ -0,0 +1,14 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package my.jpms.test;
+
+import kotlinx.datetime.*;
+
+public class InitLocalDate {
+    LocalDate newLocalDate() {
+        return new LocalDate(2025, 3, 31);
+    }
+}