[FIX] Always add raft::raft_nn_lib and raft::raft_distance_lib aliases

[trxcllnt]

Always add raft::raft_nn_lib and raft::raft_distance_lib aliases so adding raft as a submodule provides the same library targets as using it as a package.

Here's an example of the differences between using RAFT as a submodule and using it as a package (preinstalled or otherwise):

# Build raft from source
set(RAFT_COMPILE_LIBRARIES ON)
add_subdirectory(/raft/cpp)
if(TARGET raft_nn_lib) # FALSE
if(TARGET raft_distance_lib) # FALSE
if(TARGET raft::raft_nn_lib) # TRUE
if(TARGET raft::raft_distance_lib) # TRUE

# Find the RAFT package
set(raft_ROOT /raft/cpp/build)
find_package(raft)
if(TARGET raft_nn_lib) # FALSE
if(TARGET raft_distance_lib) # FALSE
if(TARGET raft::raft_nn_lib) # TRUE
if(TARGET raft::raft_distance_lib) # TRUE

The problem is exacerbated if we use CPMFindPackage, since the available raft targets will be different based on whether this is the first time we've configured or if we're re-configuring:

if(EXISTS ${CMAKE_BINARY_DIR}/_deps/raft-build)
  set(raft_ROOT ${CMAKE_BINARY_DIR}/_deps/raft-build)
endif()
# Find or build RAFT from source
CPMFindPackage(NAME raft ...)
if(TARGET raft_nn_lib) # initial configure=TRUE, reconfigure=FALSE
if(TARGET raft_distance_lib) # initial configure=TRUE, reconfigure=FALSE
if(TARGET raft::raft_nn_lib) # initial configure=FALSE, reconfigure=TRUE
if(TARGET raft::raft_distance_lib) # initial configure=FALSE, reconfigure=TRUE

[cjnolet]

@gpucibot merge