PathResolver: fix priorityClassPath

It was wrongly defined before with respect to `commandLineFor` (this is
what happens when you rely on strings instead of types!)

Author: smarter

src/dotty/tools/dotc/config/PathResolver.scala

@@ -189,7 +189,7 @@ class PathResolver(implicit ctx: Context) {
     def javaUserClassPath   = if (useJavaClassPath) Defaults.javaUserClassPath else ""
     def scalaBootClassPath  = cmdLineOrElse("bootclasspath", Defaults.scalaBootClassPath)
     def scalaExtDirs        = cmdLineOrElse("extdirs", Defaults.scalaExtDirs)
-    def priorityClassPath   = cmdLineOrElse("prioritypath", "")
+    def priorityClassPath   = cmdLineOrElse("priorityclasspath", "")
     /** Scaladoc doesn't need any bootstrapping, otherwise will create errors such as:
      * [scaladoc] ../scala-trunk/src/reflect/scala/reflect/macros/Reifiers.scala:89: error: object api is not a member of package reflect
      * [scaladoc] case class ReificationException(val pos: reflect.api.PositionApi, val msg: String) extends Throwable(msg)