CMake: Skip `find_package()` call if Abseil is already present
CMake: Skip `find_package()` call if Abseil is already present

This is similar in spirit to my [Protobuf PR #10436](https://github.com/protocolbuffers/protobuf/pull/10436) (as per [Protobuf Issue #10435](https://github.com/protocolbuffers/protobuf/issues/10435))

PiperOrigin-RevId: 484007937

protocolbuffers/utf8_range,protocolbuffers/utf8_range