[Bug #18173] Update loaded_features_index

	If $LOADED_FEATURES is changed in the just required file, also the
	index table needs to be updated before loaded_features_snapshot is
	reset.  If the snapshot is reset without updating the table, the
	name of the added feature will not be found.
	---
	 load.c                    |  1 +
	 test/ruby/test_require.rb | 17 +++++++++++++++++
	 2 files changed, 18 insertions(+)