Fix filling Main-Class attribute in manifest file of output jar

Before 5b7cee6221 JVM CLI compiler
was calling `KotlinToJVMBytecodeCompiler.compileBunchOfSources`.
`compileBunchOfSources` detected possible main classes,
and filled the Main-Class attribute in output jar
if if there was only one candidate.

After the change JVM CLI began calling
`KotlinToJVMBytecodeCompiler.compileModules`, which was not searching for a main class.

This change adds searching for main classes to `compileModules`.
We search for a main class only when one module is compiled,
and an output is written a jar file (so the change only affects JVM CLI compilation).

    #KT-32272 Fixed

compiler/tests/org/jetbrains/kotlin/cli/CustomCliTest.kt

@@ -5,10 +5,14 @@
 
 package org.jetbrains.kotlin.cli
 
+import junit.framework.Assert
 import org.jetbrains.kotlin.cli.jvm.K2JVMCompiler
 import org.jetbrains.kotlin.test.CompilerTestUtil
 import org.jetbrains.kotlin.test.TestCaseWithTmpdir
 import java.io.File
+import java.util.jar.JarFile
+
+private const val EMPTY_MAIN_FUN = "fun main() {}"
 
 class CustomCliTest : TestCaseWithTmpdir() {
     fun testArgfileWithNonTrivialWhitespaces() {
@@ -16,4 +20,33 @@ class CustomCliTest : TestCaseWithTmpdir() {
         val argfile = File(tmpdir, "argfile").apply { writeText(text, Charsets.UTF_8) }
         CompilerTestUtil.executeCompilerAssertSuccessful(K2JVMCompiler(), listOf("@" + argfile.absolutePath))
     }
+
+    fun testMainClass() {
+        val mainKt = tmpdir.resolve("main.kt").apply {
+            writeText(EMPTY_MAIN_FUN)
+        }
+        compileAndCheckMainClass(listOf(mainKt), expectedMainClass = "MainKt")
+    }
+
+    fun testMultipleMainClasses() {
+        val main1Kt = tmpdir.resolve("main1.kt").apply {
+            writeText(EMPTY_MAIN_FUN)
+        }
+        val main2Kt = tmpdir.resolve("main2.kt").apply {
+            writeText(EMPTY_MAIN_FUN)
+        }
+
+        compileAndCheckMainClass(listOf(main1Kt, main2Kt), expectedMainClass = null)
+    }
+
+    private fun compileAndCheckMainClass(sourceFiles: List<File>, expectedMainClass: String?) {
+        val jarFile = tmpdir.resolve("output.jar")
+        val args = listOf("-include-runtime", "-d", jarFile.absolutePath) + sourceFiles.map { it.absolutePath }
+        CompilerTestUtil.executeCompilerAssertSuccessful(K2JVMCompiler(), args)
+
+        JarFile(jarFile).use {
+            val mainClassAttr = it.manifest.mainAttributes.getValue("Main-Class")
+            Assert.assertEquals(expectedMainClass, mainClassAttr)
+        }
+    }
 }