Fix: Restore incremental builds with pkg-config ELPA

[AsTonyshment]

What's changed?

PR #7630 used ELPA_LIBRARIES as both a deprecated manual input and an output cached internally by FindPkgConfig. On a later configuration of the same build directory, the cached bare library name such as elpa_openmp was mistaken for a manual override. Because imported targets do not persist between CMake configurations, ELPA::ELPA was then recreated with the bare name as its imported location, causing No rule to make target 'elpa_openmp' during incremental builds.

This change moves pkg-config results to the independent ELPA_PKG prefix and aliases ELPA::ELPA to PkgConfig::ELPA_PKG. A one-time migration removes conflicting ELPA_* cache entries produced by the old implementation, allowing existing build directories to recover without deletion. The deprecated ELPA_DIR and manual include/library discovery path retains its existing search order and imported-target behavior.

Verification: repeated cmake -S . -B <build-dir> and cmake --build <build-dir> --target MODULE_HSOLVER_LCAO -j32 in an existing polluted build directory and a clean build directory; both passed twice, and the generated link command contains the resolved libelpa_openmp.so path. The deprecated manual path was also configured and built successfully with explicit ELPA_DIR, ELPA_INCLUDE_DIR, and ELPA_LIBRARIES values.

Maybe you can have a look @Growl1234, thanks! In the end the old ELPA_DIR logic will be removed when the formal version is released, so it is still an intermediate solution at present in order to make incremental compilation work properly.

Governance Notes

• Automated tests: none added; repeated same-directory configuration and focused compilation directly exercise this CMake-only regression.
• INPUT/docs changes: none; no INPUT behavior or metadata is changed.
• Core module impact: none; no files under source/ are changed.
• Exceptions requested: none.

[Copilot]

Pull request overview

This PR fixes a CMake incremental-build regression in ELPA discovery by separating pkg-config outputs from deprecated public ELPA_* inputs and ensuring ELPA::ELPA is consistently backed by a persistent pkg-config imported target across reconfigurations.

Changes:

• Migrates away from the old ELPA_* pkg-config cache entries by unsetting conflicting internal cache variables once.
• Switches pkg-config discovery to use an independent ELPA_PKG prefix and maps results back onto the public ELPA_* outputs.
• Makes ELPA::ELPA an alias of PkgConfig::ELPA_PKG when available, keeping the deprecated manual fallback path intact.

[Growl1234]

Looks good. Cannot say I like it, but OK if problem resolved. I didn't care too much about old path since we'll drop these logic and depend totally on pkg-config anyways (see this file).