Generate Automatic-Module-Name attribute (#1375) (#1400)

* Generate `Automatic-Module-Name` attribute

As it stands, Java 9 Platform Module System cannot infer module
name from shapeless' jars due to cross compilation (underscores
in `_2.xx`).
This simple change adds an `Automatic-Module-Name` attribute to
MANIFEST, making it possible to require `shapeless` modules, rather than
going through various repackaaging hacks.
A proper modularization would include definition of `module-info.java`
but that's outside the scope.

* Use shapeless Java module name for core

---------

Co-authored-by: Hubert Plociniczak <hubert.plociniczak@gmail.com>

Author: joroKr21

build.sbt

@@ -112,6 +112,7 @@ lazy val commonSettings = crossVersionSharedSources ++ Seq(
   )
 )
 
+lazy val javaModuleName = settingKey[String]("Java module name")
 lazy val jsSourceMapSettings = Def.settings(
   scalacOptions += {
     val tagOrHash =
@@ -160,7 +161,7 @@ lazy val coreTestMacros = crossProject(JSPlatform, JVMPlatform, NativePlatform)
 lazy val core = crossProject(JSPlatform, JVMPlatform, NativePlatform)
   .crossType(CrossType.Full)
   .configureCross(configureJUnit)
-  .settings(moduleName := "shapeless")
+  .settings(moduleName := "shapeless", javaModuleName := "shapeless")
   .settings(commonSettings)
   .settings(publishSettings)
   .configureCross(buildInfoSetup)
@@ -180,7 +181,7 @@ lazy val scratch = crossProject(JSPlatform, JVMPlatform, NativePlatform)
   .crossType(CrossType.Pure)
   .configureCross(configureJUnit)
   .dependsOn(core)
-  .settings(moduleName := "scratch")
+  .settings(moduleName := "scratch", javaModuleName := "shapeless.scratch")
   .settings(commonSettings)
   .settings(noPublishSettings)
 
@@ -238,7 +239,8 @@ lazy val publishSettings = Seq(
   developers := List(
     Developer("milessabin", "Miles Sabin", "", url("http://milessabin.com/blog")),
     Developer("joroKr21", "Georgi Krastev", "joro.kr.21@gmail.com", url("https://twitter.com/Joro_Kr"))
-  )
+  ),
+  packageOptions += Package.ManifestAttributes("Automatic-Module-Name" -> javaModuleName.value),
 )
 
 lazy val noPublishSettings =