arrow

ARROW-9316: [C++] Use "Dataset" instead of "Datasets"

Because we use "dataset" for ID of this module such as
libarrow_dataset.so and arrow/dataset/api.h.

Closes #7629 from kou/cpp-dataset

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Wes McKinney <wesm@apache.org>