Remove Bitcoin Cash references in CMake config files

Summary:
Change Bitcoin Cash to BCHA in CMakeLists.txt
Change PackageOptions.cmake to use description from CMakelists.txt

Test Plan:
Delete build directory, and create a new one
In fresh build directory, type CMake -GNinja ..
Check in CPackConfig.cmake that CPACK_PACKAGE_DESCRIPTION and CPACK_PACKAGE_DESCRIPTION_SUMMARY are set correctly.

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D9217