Change logic to find members of recursive types

[odersky]

The previous logic used two boolean variables to selectively create defensive copies. It was quite complicated. The new logic is simpler and copies less.

• It copies only if the same recursive type is accessed with two different prefixes. As long as the prefix stays the same,no confusion in the substRecThis is possible.
• It avoids the openedTwice logic that causes defensive copies at all points in the future. It seems that trick is no longer necessary.

Fixes #17380 by avoiding infinite recursion due to defensive copies.
Fixes #17381 as well.

[Kordyjan]

It changes the error message of #17381 to:

[error] value applyDynamic is not a member of Y, but could be made available as an extension method.
[error]
[error] The following import might fix the problem:
[error]
[error]   import reflect.Selectable.reflectiveSelectable
[error]
[error]   val y2 = y1.u()
[error]            ^^^^

The new error message seems to be expected, so I think we can consider #17381 as also fixed.