CMake config does not reexport Gau2Grid if used as external dependency

[awvwgk]

When using an installed GauXC library built against installed Gau2Grid library, the gauxc::gauxc target does not provide gau2grid::gg target.

CMake Error at /home/runner/micromamba/envs/gauxc-dev/lib/cmake/gauxc/gauxc-targets.cmake:61 (set_target_properties):
  The link interface of target "gauxc::gauxc" contains:

    gau2grid::gg

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

A possible workaround when using GauXC would be to always find GauXC as

find_package(gauxc CONFIG REQUIRED)
if(GAUXC_HAS_GAU2GRID AND NOT TARGET gau2grid::gg)
  find_package(gau2grid CONFIG REQUIRED)
endif()