Better error diagnostics and other fixes for extension methods

[odersky]

We gave misleading diagnostic before in the case where no extension
method was found since a TypeError was thrown.

Also: Avoid redundant test that prevented eta expansion.
Also: Drop support for extension_ names.

Fixes #10391
Fixes #9507
Fixes #10901

[bishabosha]

with relation to #10044 , the error now becomes a suggestion to import opaques.arrayOps:

scala> IArray(1,2,3).reverse.sorted                                                                                   
val res0: opaques.IArray[Int] = Array(1, 2, 3)

scala> IArray(1,2,3).reverse.sorted
1 |IArray(1,2,3).reverse.sorted
  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |value sorted is not a member of scala.opaques.IArray[Int], but could be made available as an extension method.
  |
  |The following import might fix the problem:
  |
  |  import opaques.arrayOps
  |

[bishabosha]

code like the issue #7056 no longer give a suggestion of a tried extension method, and instead suggest that nothing was found, e.g.

type A
type B <: A

type PartialId[X] = X match {
  case B => X
}

trait T1[T] {
  extension (t1: T) def idnt1: Any
}

given [T <: A](using PartialId[T]): T1[T] = new T1[T] {
  extension (t1: T) def idnt1: Any = ???
}

given PartialId[B] = ???

val x: B = ???
val z = x.idnt1

errors with the following:

-- [E008] Not Found Error: sandbox/cls/example.scala:19:10 ---------------------
19 |val z = x.idnt1
   |        ^^^^^^^
   |        value idnt1 is not a member of B
1 error found

which I believe is probably less helpful compared to the error in M3:

-- [E008] Not Found Error: example.scala:19:10 ---------------------------------
19 |val z = x.idnt1
   |        ^^^^^^^
   |      value idnt1 is not a member of B.
   |      An extension method was tried, but could not be fully constructed:
   |
   |          example$package.given_T1_T[T](given_PartialId_B).idnt1()
1 error found

[odersky]

The addendums for the implicit search errors were dropped by accident. Last commit brings them back.

The error message for #10044 is now also the same as it was before.