SI-6575 Plug inference leak of AbstractPartialFun

Usually this isn't needed, as partial functions can only be
defined with an expected type. But if that expected type is
and inherited method return type, the actual type of the partial
function literal is used, and the implementation detail of
`AbstractPartialFunction[A, B] with Serializable` leaks out.

After this change, the inferred types match those from
Scala 2.9.2.

ticket/6575 ~/code/scala scalac29 -Xprint:typer test/files/pos/t6575a.scala | grep def > 29.txt
ticket/6575 ~/code/scala squalac -Xprint:typer test/files/pos/t6575a.scala | grep def > 210.txt
ticket/6575 ~/code/scala diff -u 29.txt 210.txt
--- 29.txt	2012-10-28 13:51:07.000000000 +0100
+++ 210.txt	2012-10-28 13:51:20.000000000 +0100
@@ -1,7 +1,16 @@
     def foo: PartialFunction[Int,Int]
     def /*Y*/$init$(): Unit = {
-    absoverride def foo: PartialFunction[Int,Int] = ((x0$1: Int) => x0$1 match {
+    absoverride def foo: PartialFunction[Int,Int] = {
+        def <init>(): anonymous class $anonfun = {
+        final override def applyOrElse[A1 >: Nothing <: Int, B1 >: Int <: Any](x$1: A1, default: A1 => B1): B1 = (x$1: A1 @unchecked) match {
+        final def isDefinedAt(x$1: Int): Boolean = (x$1: Int @unchecked) match {
     def /*Z*/$init$(): Unit = {
-    absoverride def foo: PartialFunction[Int,Int] = ((x0$2: Int) => x0$2 match {
+    absoverride def foo: PartialFunction[Int,Int] = {
+        def <init>(): anonymous class $anonfun = {
+        final override def applyOrElse[A1 >: Nothing <: Int, B1 >: Int <: Any](x$1: A1, default: A1 => B1): B1 = (x$1: A1 @unchecked) match {
+        final def isDefinedAt(x$1: Int): Boolean = (x$1: Int @unchecked) match {
     def /*Comb*/$init$(): Unit = {
-    absoverride def foo: PartialFunction[Int,Int] = ((x0$3: Int) => x0$3 match {
+    absoverride def foo: PartialFunction[Int,Int] = {
+        def <init>(): anonymous class $anonfun = {
+        final override def applyOrElse[A1 >: Nothing <: Int, B1 >: Int <: Any](x$1: A1, default: A1 => B1): B1 = (x$1: A1 @unchecked) match {
+        final def isDefinedAt(x$1: Int): Boolean = (x$1: Int @unchecked) match {
Wider use and a new variant of typedPos.

It's safe to replace `localTyper.typed(atPos(pos)(tree))` with
`localTyper.typedPos(pos)(tree)` given that we're all in the
same cake and we'll get to the same `atPos`.
Fixes SI-5031 for separate compilation scenario.

When you have a conflicting member in package object and normal package that share the same namespace we remove the latter ClassSymbol from the scope. Now, this has an unpleasant consequence that companionClass/companionModule/companionSymbol no longer work correctly as they rely on finding the correspondent symbol using decls of the owner.
This fixes the problem of SI-5031 for separate compilation. Why the above change matters for finding foo.bar.Foo? Because when parsing the class we needed information about the static module (and we have the correct module symbol when completing the info). It's just that 043ce6d0565c9d5d960 relied on no longer valid assumptions. So we were getting NoSymbol and sym.exist was failing.

Obviously a more complete solution would be better if we didn't rely on the scope but that's too big to change for 2.10.0.
Use Typed rather than .setType

In response to pull request feedback.
SI-6581 fixed by inlining `Actor.self`.

This avoids the necessary type cast that was preventing leakage of internal migration classes.

Review by @phaller
SI-6578 Harden against synthetics being added more than once.

Don't add synthetic methods if they already contain synthetic
methods from the set we're about to add.
Merge pull request #1530 from retronym/ticket/6575-2

SI-6575 Plug inference leak of AbstractPartialFunction
Merge pull request #1532 from hubertp/2.10.0-wip-issue/5031

Fixes SI-5031 for separate compilation scenario.
Merge pull request #1536 from vjovanov/issues/SI-6581

SI-6581 fixed by inlining `Actor.self`.
Merge pull request #1543 from adriaanm/paulp-add-synthetics-once

SI-6578 Harden against synthetics being added more than once.