Support installing the package with CMake FetchContent

Example snippet:
```cmake
set(BUILD_SHARED_LIBS ON)
FetchContent_Declare(
aws-lambda-cpp
GIT_REPOSITORY https://github.com/awslabs/aws-lambda-cpp.git
GIT_TAG v0.2.10
GIT_SHALLOW TRUE
OVERRIDE_FIND_PACKAGE TRUE
)
unset(BUILD_SHARED_LIBS CACHE)

find_package(aws-lambda-cpp)

target_link_libraries(${PROJECT_NAME} PRIVATE aws-lambda-cpp::aws-lambda-runtime)

aws_lambda_package_target(${PROJECT_NAME} NO_LIBC)
```

Currently fails with the following error:

```shell
CMake Error at CMakeLists.txt:48:
Parse error. Expected a command name, got unquoted argument with text
"aws-lambda-cpp::aws_lambda_package_target".
```