Do not include .class files in the packaged .jars.

They are supposed to be provided by the host JDK.

To test that the test suite passes with the host JDK .class files
only, we use `exportJars := true`.

Author: sjrd

build.sbt

@@ -22,6 +22,11 @@ lazy val root: Project = project.in(file(".")).
   settings(
     name := "scalajs-java-time",
 
+    mappings in (Compile, packageBin) ~= {
+      _.filter(!_._2.endsWith(".class"))
+    },
+    exportJars := true,
+
     publishMavenStyle := true,
     publishTo := {
       val nexus = "https://oss.sonatype.org/"