QCalendar: port registry from QHash to QFlatMap

Unlike many other uses of QFlatMap, this actually promises to hit the
QFlatMap sweet spot, and we're using all its levers.

To wit:

- Enable lookup via QAnyStringView through a transparent comparator
  without having to convert the QAnyStringView to QString first. This
  is impossible with QHash, because qHash() values are not consistent
  for different string types: u"a"_qsv == "a"_L1, but qHash(u"a"_qsv)
  != qHash("a"_L1).  The relational operators don't have this
  consistency problem, and they also implement case-insensitive
  comparison, another thing qHash lacks.

- Pick different types of containers for keys and values. For the
  keys, use QStringList, which makes availableCalendars() trivial and
  extremely fast. We can reserve() the flat_map to limit the effect of
  reallocations, because we have a pretty good idea about how many
  entries we'll have. For the values, use the same container as for
  byID. This might be better with QVarLengthArray, but that's for
  another patch.

- Fix the double lookup in registerBackendLockHeld() by using
  try_emplace() instead of contains() + insert().

To enable QFlatMap's full potential, we still need to teach
ensurePopulated() to create the two containers for the QFlatMap
unordered and let QFlatMap sort them in one go, but that, too, is for
another patch.

Change-Id: I7fe4f3f7596e9b234696fbc8e467128b85629f8a
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

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