Move SymDenotation#proxy to SymbolLoader#proxy

It's only ever used on SymbolLoaders, and more importantly it means that
a proxy to a SymbolLoader is now also a SymbolLoader which we'll use to
enforce restrictions on which completers are allowed to set
`privateWithin` in a later commit.

Author: smarter

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -2155,13 +2155,6 @@ object SymDenotations {
     private[this] var mySourceModuleFn: Context => Symbol = NoSymbolFn
     private[this] var myModuleClassFn: Context => Symbol = NoSymbolFn
 
-    /** A proxy to this lazy type that keeps the complete operation
-     *  but provides fresh slots for scope/sourceModule/moduleClass
-     */
-    def proxy: LazyType = new LazyType {
-      override def complete(denot: SymDenotation)(implicit ctx: Context) = self.complete(denot)
-    }
-
     /** The type parameters computed by the completer before completion has finished */
     def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeParamInfo] =
       if (sym.is(Touched)) Nil // return `Nil` instead of throwing a cyclic reference

compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala

@@ -200,6 +200,7 @@ object SymbolLoaders {
    */
   class PackageLoader(_sourceModule: TermSymbol, classPath: ClassPath)
       extends SymbolLoader {
+    override def sourceFileOrNull: AbstractFile = null
     override def sourceModule(implicit ctx: Context): TermSymbol = _sourceModule
     def description(implicit ctx: Context): String = "package loader " + sourceModule.fullName
 
@@ -302,18 +303,25 @@ object SymbolLoaders {
 /** A lazy type that completes itself by calling parameter doComplete.
  *  Any linked modules/classes or module classes are also initialized.
  */
-abstract class SymbolLoader extends LazyType {
-
+abstract class SymbolLoader extends LazyType { self =>
   /** Load source or class file for `root`, return */
   def doComplete(root: SymDenotation)(implicit ctx: Context): Unit
 
-  def sourceFileOrNull: AbstractFile = null
+  def sourceFileOrNull: AbstractFile
 
   /** Description of the resource (ClassPath, AbstractFile)
    *  being processed by this loader
    */
   def description(implicit ctx: Context): String
 
+  /** A proxy to this loader that keeps the doComplete operation
+   *  but provides fresh slots for scope/sourceModule/moduleClass
+   */
+  def proxy: SymbolLoader = new SymbolLoader {
+    export self.{doComplete, sourceFileOrNull}
+    def description(implicit ctx: Context): String = "proxy to ${self.description}"
+  }
+
   override def complete(root: SymDenotation)(implicit ctx: Context): Unit = {
     def signalError(ex: Exception): Unit = {
       if (ctx.debug) ex.printStackTrace()