Ensure that read-only methods don't use exclusive capabilities

[odersky]

Ensure that read-only methods don't use non-local exclusive capabilities

Fixes #24310

[Linyxus]

There is no "actual" data races in the original example so I suggest adding this example too:

import language.experimental.captureChecking
import language.experimental.separationChecking
import caps.*

class Ref extends Mutable:
  private var value: Int = 0
  def get(): Int = value
  update def set(v: Int): Unit = value = v

class Matrix(val f: () => Int) extends Mutable:
  self: Matrix^ =>
    def run() = f() // <- note the missing update
    update def add(): Unit = ()

@main def test =
  val r: Ref^ = Ref()
  def inc(): Int =
    val old = r.get()
    r.set(old + 1)
    old
  val m4: Matrix^ = Matrix(inc)

  def par(f1: () => Int, f2: () => Int): Unit =
    println(s"par results: ${f1()} and ${f2()}")

  def g(m: Matrix^): Unit =
    par(m.run, m.run) // <- should be rejected

  g(m4) // ok, but should not