Propagate main class defined from the sbt build

[tindzk]

As per #716, the detection of main classes is restricted to classes defined within the project. In the case of Play, the sbt project already sets the main class correctly:

[play-scala-seed] $ show mainClass
[info] Some(play.core.server.ProdServerStart)

It would be useful to use this value and set it in the Bloop configuration file such that it does not have to be passed as a command-line parameter.

[jvican]

@tindzk Do you feel like taking a stab at this? Should take only a few minutes 😄

[tindzk]

Sure, I will have a look.

[jvican]

@tindzk Great! The configuration field for the main class is already there, we just need to propagate it with something like:

        val jvmConfig = Config.JvmConfig(Some(javaHome.toPath), javaOptions.toList)
        val mainClass = Keys.mainClass.in(config).value
        Config.Platform.Jvm(jvmConfig, mainClass)

where config == Keys.configuration.value. We should probably also do the same for Gradle, Maven and mill if we're not doing it there, and add tests for gradle and sbt (in sbt, you can add a check in one of the scripted tests that checks the contents of the JSON configuration files).

[jvican]

We cannot depend on mainClass directly in sbt because it forces the compilation of projects (internally, mainClass depends on discoveredMainClasses which triggers compilation during bloopInstall, which makes the process slow).

As there is no way to avoid that in sbt, I've introduced a bloopMainClass setting that can be set just as mainClass and that users can use to avoid duplicating the main class every time they want to run an application. There is no better solution than this.