title: Not able to cross publish my sbt plugin for multiple Scala versions
body: <p>I have the following sbt file</p>
  
  <pre><code>lazy val root = (project in file(".")).
    settings(
      inThisBuild(List(
        sbtPlugin := true, 
        organization := "com.foo",
        crossScalaVersions := Seq("2.11.2", "2.12.0"),
        version      := "1.0.0"
      )),
      name := "myplugin",
      libraryDependencies ++= Seq(
        "org.scala-lang.modules" %% "scala-xml" % "1.0.6",
        "com.typesafe" % "config" % "1.3.3"
      )
    )
  </code></pre>
  
  <p>Now I can easily do <code>sbt publishLocal</code> and I see that it generates a jar file in the <code>.ivy2/local/com.foo/myplugin/scala_2.12/sbt_1.0/1.0.0/jars/</code></p>
  
  <p>but if I do a</p>
  
  <pre><code>sbt +publishLocal
  </code></pre>
  
  <p>I get an error</p>
  
  <pre><code>[error] Modules were resolved with conflicting cross-version suffixes in ProjectRef(uri("file:/Users/user/myplugin/"), "root"):
  [error]    org.scala-lang.modules:scala-xml _2.11, _2.12
  [error]    org.scala-lang.modules:scala-parser-combinators _2.11, _2.12
  [error] java.lang.RuntimeException: Conflicting cross-version suffixes in: org.scala-lang.modules:scala-xml, org.scala-lang.modules:scala-parser-combinators
  [error]         at scala.sys.package$.error(package.scala:27)
  [error]         at sbt.librarymanagement.ConflictWarning$.processCrossVersioned(ConflictWarning.scala:39)
  [error]         at sbt.librarymanagement.ConflictWarning$.apply(ConflictWarning.scala:19)
  [error]         at sbt.Classpaths$.$anonfun$ivyBaseSettings$64(Defaults.scala:1995)
  [error]         at scala.Function1.$anonfun$compose$1(Function1.scala:44)
  [error]         at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:39)
  [error]         at sbt.std.Transform$$anon$4.work(System.scala:66)
  [error]         at sbt.Execute.$anonfun$submit$2(Execute.scala:262)
  </code></pre>
  
  <p>My expectation was that SBT will do the compilation and publishing twice and each time it will pick the right jars for the right Scala version. Why does it say that there is a conflict?</p>
  
  <p>My end goal is to make SBT publish multiple jar files one for each scala version in my <code>crossScalaVersions</code> list.</p>