* cmake: use target aliases from local Google Test checkout.

`ABSL_FIND_GOOGLETEST` is a dependent option, it is always `OFF` when
`ABSL_USE_EXTERNAL_GOOGLETEST` is `OFF` (i.e. when Google Test is
checked out locally).

`GTest::` target aliases are to be added only if `find_package(GTest)`
has been actually used.

* cmake: clarify adding GTest:: aliases.