Use cpm_find_package to check if a package added

[black-desk]

If I have a project, which has:

1. /CMakeLists.txt:

   cmake_minimum_required(VERSION 3.29)
   project(test)
   include(./cmake/CPM.cmake)
   add_subdirectory(subdir)
   cpmfindpackage(
     NAME
     nlohmann_json
     VERSION
     3.11.2
     URL
     "https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz"
     EXCLUDE_FROM_ALL
     ON)
   
   if(NOT TARGET nlohmann_json::nlohmann_json)
     message(FATAL_ERROR "BUG")
   endif()

2. /subdir/CMakeLists.txt

   cpmfindpackage(
     NAME
     nlohmann_json
     VERSION
     3.11.2
     URL
     "https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz"
     EXCLUDE_FROM_ALL
     ON)

When nlohmann_json is first founded by find_package,
CPM.cmake register this package and later call to CPMFindPackage will
not actually perform a find_package again, which leads to this buggy
behavior.

As CPM write a FindXXX.cmake file for packages added by CPM to pervert
find_package to get local package, I think directly use cpm_find_package
to check if a package is already added is good enough.

[black-desk]

https://github.com/black-desk/cpm-test1

[TheLartians]

Thanks for the PR! I can't quite follow all the implications atm, but your reasoning seems sound and tests pass, so I'm happy with merging this.