Warn universal extensions on opaque type

[som-snytt]

Compiler version

3.6.2

Minimized code

class C
object C:
  extension (c: C) def equals(that: Any): Boolean = false // warn

object X:
  class C
  opaque type D <: C = C
  object D:
    extension (d: D) def equals(that: Any): Boolean = false // nowarn

Output

-- [E194] Potential Issue Warning: .../no-extension.scala:12:23 -
12 |  extension (c: C) def equals(that: Any): Boolean = false
   |                       ^
   |Extension method equals will never be selected
   |because C already has a member with the same name and compatible parameter types.

Expectation

Complain about defining extension methods which can't be invoked because they are universally available.

The loophole for opaque types is too wide.

Noticed at #17821.