Warn trivial recursion with module prefix

[som-snytt]

Fixes #23277

First, loosen the test for whether to warn for module.recur(). The this assignment has a RHS that is not pure (because of module init) but isStable (and not significant with respect to the warning).

Second, avoid emitting such a this assignment if the prefix is the enclosing module. The existing code checks for a prefix that is Enclosing.this.

Other "aliasing" prefixes are not considered, such as a method with result this.type, since they could execute arbitrary intended effects.

[som-snytt]

As pointed out on the ticket,

def apply(i: Int, s: String): Two = Two(i, s) // warn because unprefixed Two is deemed pure

Also pondering rhs.tpe.isStable vs tpd.isIdempotentPath(rhs). Also whether to revert that change if the second fix is accepted, since I can't trick it into using that code path.

[tgodzik]

LGTM!