Provide ALIAS targets to unify the usage

If googletest is made part of the dependent's build tree (via FetchContent), then usage is different compared to using it from an install location (via find_package).

Ever since FetchContent has been introduced, the example has been using googletest. See here.

Usage differences in code:

# make googletest part of hte build tree
include(FetchContent)
FetchContent_Declare(googletest
GIT_REPOSITORY https://github.com/google/googletest.git
GIT_TAG master)
FetchContent_MakeAvailable(googletest)

# no ALIAS target, must use non-namespaced target instead
target_link_libraries(my_target PRIVATE gtest_main)

# ----

# use googletest from an install location
find_package(GTest REQUIRED CONFIG)

# the install(EXPORT) command namespaces the targets
target_link_libraries(my_target PRIVATE GTest::gtest_main)
I see from another issue (#3040 (comment)), that due to CMake 3.19 deprecating <2.8.12 there are also plans to set that as the minimum version and conveniently 2.8.12 is also the version that introduced the ALIAS form for add_library.