Use an alias target for the root add_library

[azeey]

Without this patch, doing find_package(gz-utils) (or any vendor package) multiple times in a single CMakeLists file fails with an error indicating that cmake cannot create the imported target because another target with the same name already exists.

Test:

cmake_minimum_required(VERSION 3.10)
project(test_gz_vendor)

find_package(gz_utils_vendor REQUIRED)
find_package(gz-utils REQUIRED)
find_package(gz-utils REQUIRED)

Once approved, I'll apply the change to the template and regenerate all vendor packages.

[mjcarroll]

To prevent future issues, we could also wrap the whole thing in the CMake if (NOT defined $FOO) set(FOO TRUE) endif() pattern, but this seems safe for now, provided we don't add more.

[azeey]

To prevent future issues, we could also wrap the whole thing in the CMake if (NOT defined $FOO) set(FOO TRUE) endif() pattern, but this seems safe for now, provided we don't add more.

I didn't want to do it wholesale like that, if possible, because the requested components might be different between the two invocations of find_package. I thought about checking if each target existed before creating the alias, but that doesn't seem to be necessary for aliases.