Make benchmarks depend only on library classpath

Testing that our benchmark program don't need to link against the compiler.
Warning: This might make performance incomparable with earlier (similarly to
other current changes.

Author: Blaisorblade

project/Build.scala

@@ -302,7 +302,7 @@ object Build {
   lazy val commonBenchmarkSettings = Seq(
     outputStrategy := Some(StdoutOutput),
     mainClass in (Jmh, run) := Some("dotty.tools.benchmarks.Bench"), // custom main for jmh:run
-    javaOptions += "-DBENCH_CLASS_PATH=" + Attributed.data((fullClasspath in Compile).value).mkString("", ":", "")
+    javaOptions += "-DBENCH_CLASS_PATH=" + Attributed.data((fullClasspath in (`dotty-library`, Compile)).value).mkString("", ":", "")
   )
 
   // sbt >= 0.13.12 will automatically rewrite transitive dependencies on