[C++] FindArrowDataset requires Parquet

[asfimport]

Parquet is an optional dependency of the dataset layer, but it appears that FindArrowDataset.cmake requires Parquet unconditionally.

Example:
https://app.travis-ci.com/github/apache/arrow/jobs/548648734#L3513

  Could NOT find Parquet (missing: PARQUET_INCLUDE_DIR PARQUET_LIB_DIR

  PARQUET_SO_VERSION)

Call Stack (most recent call first):

  /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:393 (_FPHSA_FAILURE_MESSAGE)

  cmake_modules/FindParquet.cmake:115 (find_package_handle_standard_args)

  cmake_modules/FindArrowDataset.cmake:47 (find_package)

  CMakeLists.txt:432 (find_package)

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++] CMake's find_package(Parquet) does not find Parquet with Arrow 3.0.0 (is related to)

_{Note: This issue was originally created as ARROW-14793. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @kou

[asfimport]

Kouhei Sutou / @kou:
We should also fix this in ARROW-12175 work.