Fix scala#2186: Synchronize classpath handling with Scala 2.12

This commit is a very crude port of the classpath handling as it exists
in the 2.12.x branch of scalac (hash: 232d95a),
this replaces the existing Classpath code that was adapted from scalac
years ago.

This code was written by Grzegorz Kossakowski, Michał Pociecha, Lukas
Rytz, Jason Zaugg and other scalac contributors, many thanks to them!

For more information on this implementation, see the description of the
PR that originally added it to scalac: scala#4060

Changes made to the copied code to get it to compile with dotty:
- Rename scala.tools.nsc.util.ClassPath to dotty.tools.io.ClassPath
- Rename scala.tools.nsc.classpath.* to dotty.tools.dotc.classpath.*
- Replace "private[nsc]" by "private[dotty]"
- Changed `isClass` methods in FileUtils to skip Scala 2.11
  implementation classes (needed until we stop being retro-compatible with
  Scala 2.11)

I also copied PlainFile.scala from scalac to get access to
`PlainNioFile`.

Author: smarter

test/xsbt/ScalaCompilerForUnitTesting.scala

@@ -173,7 +173,7 @@ class ScalaCompilerForUnitTesting(nameHashing: Boolean, includeSynthToNameHashin
   private def prepareCompiler(outputDir: File, analysisCallback: AnalysisCallback, classpath: String = ".") = {
     val args = Array.empty[String]
 
-    import dotty.tools.dotc._
+    import dotty.tools.dotc.{Compiler, Driver}
     import dotty.tools.dotc.core.Contexts._
 
     val driver = new Driver {