All other compiler plugins accept "pkg1.pkg2.Class1" classes notation
and correctly match against it
But Assignment plugin used `ClassId` on such qualified names and got
`ClassId("/pkg1.pkg2.Class1")` instead of `ClassId("pkg1/pkg2/Class1")`,
and that lead to unexpected problems
This commit fixes that by using `FqName` instead of `ClassId`
^KT-57406 Fixed
`candidateSymbol` has any reasonable meaning only for references with
not completed candidate, so this property is moved from FirNamedReference
to new node FirNamedReferenceWithCandidateSymbol, which has real
implementation only in :resolve module (`FirNamedReferenceWithCandidate`)
Roman Golyshev