Clean up `iterable.h`:

* Remove public `IteratorT<Iterable>`. It is used only internally.

* Remove support for `Dependency` from `HasMovableElements` and
  `MaybeMakeMoveIterator`. The iterable is only a value type, possibly const,
  or a reference, without interpreting pointers, `std::unique_ptr` etc.

  This is rarely needed, and this makes the traits conceptually simpler,
  more similar to lower level traits.

  (The only usage of a type providing an iterable through `Dependency`,
  as opposed to an iterable itself, is in some pending code. That code can
  resolve `Dependency` itself.)

* Let `ElementType` reflect the semantics of `MaybeMakeMoveIterator()`,
  returning the right kind of reference or value.

  This propagates to `IsIterableOf`. In particular if the element type is
  movable but not copyable, the iterable must support moving elements out.

* Add `IsIterableOfPairs`. It is the more basic version of
  `IsIterableOfPairsWithAssignableValues`, where values are read rather than
  being assigned to.

* Add `IterableHasSize`.

PiperOrigin-RevId: 882968287