external gtest dependency importing (cmake)

[nicktohzyu]

CMake expects the targets gtest gtest_main gmock (at cmake/SapiDeps.cmake:48, while externally installed googletest usually exposes these under the GTest namespace, eg as GTest::gtest instead.

Relatedly, set(ABSL_FIND_GOOGLETEST OFF) in cmake/abseil-cpp.cmake makes it hard to use external gtest for the absl subdependency.

Was it intentional to not support external gtest? I've found that one workaround for these is to add aliases for the gtest targetes, and to set ABSL_FIND_GOOGLETEST=ON.

[hiSandog]

The scope would be easier to review if the first version defined a small acceptance contract for external gtest dependency importing. I would call out the default behavior, the migration/backwards-compatibility story, and one or two cases that are intentionally out of scope so the implementation does not grow beyond the original request.