Incorrect use of FindThreads in cmake package config file

[K-ballo]

The generated cmake package config file does a direct inclusion of the FindThread module:

abseil-cpp/CMake/abslConfig.cmake.in

Line 3 in 1a02b7a

include(FindThreads)

This results in the following cmake developer warning:

 CMake Warning (dev) at /usr/local/lib/python3.6/dist-packages/cmake/data/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
  The package name passed to `find_package_handle_standard_args` (Threads)
  does not match the name of the calling package (absl).  This can lead to
  problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.

The proper way to consume find modules is via find_package(<NAME>), which interacts with the module by setting and reading various variables prefixed with <NAME> (hence the above warning).

Furthermore, the find_dependency macro was created to wrap find_package calls in package configuration files to correctly handle QUIET and REQUIRED arguments given to to the original call.

Summing up, rather than directly including the FindThreads module, the cmake package file should instead do:

include(CMakeFindDependencyMacro)
find_dependency(Threads)