Fix scala#4230: Handle import in the REPL correctly

We used to collect imports from the parsed tree and insert them as
parsed trees at the top of the REPL wrapper object. This caused members
shadowing issues.

We now introduce a phase in the REPL compiler that collects imports
after type checking and store them as typed tree. We can then create a
context with its imports set in the correct order and use it to compile
future expressions.

Author: allanrenucci

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -317,7 +317,7 @@ class ReplDriver(settings: Array[String],
       for {
         objectIndex <- 1 to state.objectIndex
         imp <- state.imports.getOrElse(objectIndex, Nil)
-      } out.println(imp.show(state.context))
+      } out.println(SyntaxHighlighting(imp.show(state.context)))
       state
 
     case Load(path) =>