Improve history hygiene in REPL

[som-snytt]

REPL 2 warns about unintended shadowing of companions:

Welcome to Scala 2.13.4 (OpenJDK 64-Bit Server VM, Java 15).
Type in expressions for evaluation. Or try :help.

scala> class C ; object C { val c = 42 }
class C
object C

scala> class C
class C
warning: previously defined object C is not a companion to class C.
Companions must be defined together; you may wish to use :paste mode for this.

There are similar cases where "historical" values are used unintentionally:

Starting scala3 REPL...
scala> enum E(n: Int) extends Enum[E] { private final val k = 42 ; case X extends E(k) }
1 |enum E(n: Int) extends Enum[E] { private final val k = 42 ; case X extends E(k) }
  |                                                                             ^
  |                                                              Not found: k

scala> {
     | import E._
     | enum E(n: Int) extends Enum[E] { case X extends E(k) }
     | object E { final val k = 42 }
     | }
// defined class E
// defined object E

scala> enum E(n: Int) extends Enum[E] { private final val k = 42 ; case X extends E(k) }
// defined class E

scala>

Probably any indirect reference to identifiers that are no longer in "REPL scope" is unintended.

Some users expect expressions to be automatically re-evaluated (re-compiled) whenever subexpressions are re-defined. REPL could warn that it doesn't work that way. (Or offer a mode where it does work that way, a matter for a different feature request.)