Will be needed in subsequent commits for the annotation checker.
`findExpectForActual` is not suitable for the checker because
`findExpectForActual` searches expect class with same `ClassId` for
actual class containing current member (see method
`findClassifiersFromModule`), which is:
1. Unnecessary, since we already have expect class in annotation checker
2. Incorrect when class actualized via typealias and `ClassId` is
different.
So, it is needed to extract logic of searching potential expects
for actual member and already known expect class.
^KT-60668
^KT-60936
Roman Efremov
d614f06259