Findzstd.cmake conflicts with CMake-generated zstd config module

[ziyao233]

When importing LLVM 20.1.4 before zstd 1.5.7 (built with CMake) in CMake config mode, CMake throws strange errors,

# Example
cmake_minimum_required(VERSION 3.10 FATAL_ERROR)

project(test LANGUAGES C CXX)

find_package(LLVM CONFIG)
find_package(zstd CONFIG)

-- The C compiler identification is Clang 20.1.4
-- The CXX compiler identification is Clang 20.1.4
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at /lib64/cmake/zstd/zstdTargets.cmake:42 (message):
  Some (but not all) targets in this export set were already defined.

  Targets Defined: zstd::libzstd_shared

  Targets not yet defined: zstd::libzstd

Call Stack (most recent call first):
  /lib64/cmake/zstd/zstdConfig.cmake:42 (include)
  CMakeLists.txt:6 (find_package)


-- Configuring incomplete, errors occurred!

It seems llvm/cmake/modules/Findzstd.cmake defines only a subset of targets defined in zstdTargets.cmake (only zstd::libzstd_shared but not zstd::libzstd) thus messes zstd's cmake script up, which considers an incomplete importing happens.

[ziyao233]

One affected project is qt6-tools, where both libclang and zstd are used and LLVM is imported before zstd.