Improve capture set inference for lexically closest capability search

_Extracted / follow-up from #1194_

The following program still does not work, the capability set is not inferred:
```scala
interface Greet[T] { def sayHello(x: T): Unit }

def helloWorld() = try {
do sayHello(box {})
// ⚠️error: Effects need to be fully known, but effect Greet[() => Unit at ?C]'s type parameter(s) ?10319 could not be inferred.
// info: Did you mean to use an operation of the concrete interface Greet[() => Unit at {}]?
} with Greet[=> Unit at {}] {
def sayHello(x) = { println("Hello!"); resume(()) }
}
```