Prevent using item repositories without locking

Convert struct ItemRepositoryFor into a class to make repo() private in
template specializations. Convert itemRepositoryOp() function template
into op() static member function inside struct LockedItemRepository.
Make LockedItemRepository a friend of each ItemRepositoryFor
specialization to ensure that unlocked item repositories are accessed in
a single place and prevent accidental access without locking.

Add special-purpose safe access functions to the template
specializations as needed: to initialize declaration repositories and to
let IndexedIdentifier and IndexedQualifiedIdentifier share a mutex.