Add CMake ALIAS targets

We follow the findProtobuf.cmake naming convention
to ease the use of protobuf as cmake subproject

cmake/libprotobuf-lite.cmake

@@ -65,3 +65,4 @@ set_target_properties(libprotobuf-lite PROPERTIES
     VERSION ${protobuf_VERSION}
     OUTPUT_NAME ${LIB_PREFIX}protobuf-lite
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")
+add_library(protobuf::libprotobuf-lite ALIAS libprotobuf-lite)

cmake/libprotobuf.cmake

@@ -128,3 +128,4 @@ set_target_properties(libprotobuf PROPERTIES
     VERSION ${protobuf_VERSION}
     OUTPUT_NAME ${LIB_PREFIX}protobuf
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")
+add_library(protobuf::libprotobuf ALIAS libprotobuf)

cmake/libprotoc.cmake

@@ -228,3 +228,4 @@ set_target_properties(libprotoc PROPERTIES
     VERSION ${protobuf_VERSION}
     OUTPUT_NAME ${LIB_PREFIX}protoc
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")
+add_library(protobuf::libprotoc ALIAS libprotoc)

cmake/protoc.cmake

@@ -4,3 +4,4 @@ set(protoc_files
 
 add_executable(protoc ${protoc_files})
 target_link_libraries(protoc libprotobuf libprotoc)
+add_executable(protobuf::protoc ALIAS protoc)