CMake: Remove FindPEXSI.cmake

[Growl1234]

This PR removes the legacy FindPEXSI.cmake module and switches PEXSI discovery to the CMake config package provided by PEXSI itself.

The main changes are:

• Require PEXSI 2.0.0 or newer for CMake builds with ENABLE_PEXSI=ON.
• Use find_package(PEXSI 2.0.0 REQUIRED CONFIG) instead of the custom find module.
• Link ABACUS against the imported PEXSI::PEXSI target, so PEXSI include directories, compile definitions, and transitive dependencies are propagated through target usage requirements.
• Remove the manual PEXSI / SuperLU_DIST / ParMETIS / METIS library and include variables from the CMake build.
• Update CI setup to provide the required package prefixes through CMAKE_PREFIX_PATH.
• Update the installation documentation to describe the new CMAKE_PREFIX_PATH based workflow, and mention that Spack can be used to install PEXSI and its dependencies.

This fundamentally resolves #7572, as all environment variables except CMAKE_PREFIX_PATH are removed.

Possible breaking change

For CMake builds, the old variables PEXSI_DIR, ParMETIS_DIR, and SuperLU_DIST_DIR are no longer available for ABACUS CMake, because the custom module has been removed. Users building ABACUS with PEXSI support should now make sure that the PEXSI CMake config package is discoverable by adding PEXSI and its dependency prefixes to CMAKE_PREFIX_PATH if they're not installed at standard place like /usr or /usr/local.

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[ZhouXY-PKU]

Dockerfile.intel is also need to get modified.

[Growl1234]

Dockerfile.intel is also need to get modified.

Thanks for the hint. Since CI test "Build extra component with Intel toolchain" has passed, I don't think it really needs modification due to the change here.

[QuantumMisaka]

PEXSI config-package compatibility review.

[QuantumMisaka]

LGTM, and cmake_dependent_option fix need another PR