Make package command output name fallback to actual compiled main class name if there's no default main class override present

Author: Gedochao

modules/cli/src/main/scala/scala/cli/commands/Package.scala

@@ -188,6 +188,8 @@ object Package extends ScalaCommand[PackageOptions] {
           .map(_.stripSuffix("_sc"))
           .map(_ + extension)
       }
+      .orElse(build.retainedMainClass.map(_.stripSuffix("_sc") + extension).toOption)
+      .orElse(build.sources.paths.collectFirst(_._1.baseName + extension))
       .getOrElse(defaultName)
     val destPath      = os.Path(dest, Os.pwd)
     val printableDest = CommandUtils.printablePath(destPath)

modules/integration/src/test/scala/scala/cli/integration/PackageTestDefinitions.scala

@@ -77,7 +77,7 @@ abstract class PackageTestDefinitions(val scalaVersionOpt: Option[String])
         stdout = os.Inherit
       )
 
-      val outputName = if (Properties.isWin) "app.bat" else "app"
+      val outputName = if (Properties.isWin) "simple.bat" else "simple"
       val launcher   = root / outputName
 
       expect(os.isFile(launcher))
@@ -109,7 +109,7 @@ abstract class PackageTestDefinitions(val scalaVersionOpt: Option[String])
         stdout = os.Inherit
       )
 
-      val outputName = if (Properties.isWin) "app.bat" else "app"
+      val outputName = if (Properties.isWin) "hello.bat" else "hello"
       val launcher   = root / outputName
 
       val output = os.proc(launcher.toString).call(cwd = root).out.text().trim
@@ -171,7 +171,7 @@ abstract class PackageTestDefinitions(val scalaVersionOpt: Option[String])
         stdout = os.Inherit
       )
 
-      val outputName = if (Properties.isWin) "app.bat" else "app"
+      val outputName = if (Properties.isWin) "hello.bat" else "hello"
       val launcher   = root / outputName
 
       val output = os.proc(launcher.toString).call(cwd = root).out.text().trim
@@ -324,7 +324,7 @@ abstract class PackageTestDefinitions(val scalaVersionOpt: Option[String])
         os.rel / fileName ->
           s"""|//> using jsHeader "$jsHeaderNewLine"
               |//> using jsMode "release"
-              |             
+              |
               |object Hello extends App {
               |  println("Hello")
               |}
@@ -480,7 +480,7 @@ abstract class PackageTestDefinitions(val scalaVersionOpt: Option[String])
         stdout = os.Inherit
       )
 
-      val outputName = if (Properties.isWin) "app.bat" else "app"
+      val outputName = if (Properties.isWin) "Main.bat" else "Main"
       val launcher   = root / outputName
 
       val output = os.proc(launcher.toString).call(cwd = root).out.text().trim

website/docs/cookbooks/scala-package.md

@@ -8,7 +8,7 @@ The JAR file only contains the byte code that’s generated from your source cod
 
 As an example, the following snippet contains a short application to detect the OS:
 ```scala title=DetectOsApp.scala
-object DetectOSApp extends App  {
+object DetectOsApp extends App  {
     def getOperatingSystem(): String = {
         val os: String = System.getProperty("os.name")
         os