Add find_package for eigen

[nathanielerowe]

This PR is a fix for building with spack. Historically, we have relied on transitive dependencies even when we need direct dependencies. This gets rid of the following error which occurs at build time:

CMake Error at /daq/software/spack_packages/larreco/10.01.12/linux-almalinux9-x86_64_v2-gcc-12.1.0-r4nhaewmkymcijo6kzkl6ux2vqkxfqaj/lib/larreco/cmake/larrecoTargets.cmake:186 (set_target_properties):
  The link interface of target "larreco::Voronoi_DCEL" contains:

    Eigen3::Eigen

  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.

Call Stack (most recent call first):
  /daq/software/spack_packages/cetmodules/3.26.00/linux-almalinux9-x86_64_v2-gcc-12.1.0-6wueazjwav57juntb6cy4v2kr3fhaxkq/Modules/compat/art/CetCMPCleaner.cmake:71 (_include)
  /daq/software/spack_packages/larreco/10.01.12/linux-almalinux9-x86_64_v2-gcc-12.1.0-r4nhaewmkymcijo6kzkl6ux2vqkxfqaj/lib/larreco/cmake/larrecoConfig.cmake:169 (include)
  /daq/software/spack_packages/cetmodules/3.26.00/linux-almalinux9-x86_64_v2-gcc-12.1.0-6wueazjwav57juntb6cy4v2kr3fhaxkq/Modules/private/CetOverrideFindPackage.cmake:180 (_find_package)
  CMakeLists.txt:61 (find_package)

[kjplows]

LGTM -- for the online, having more direct dependencies is not an issue

[kjplows]

trigger build LArSoft/lar*@LARSOFT_SUITE_v10_09_00

[FNALbuild]

✔️ CI build for LArSoft Succeeded on slf7 for c14:prof -- details available through the CI dashboard

[FNALbuild]

✔️ CI build for LArSoft Succeeded on slf7 for e26:prof -- details available through the CI dashboard

[FNALbuild]

❌ CI build for ICARUS Failed at phase build ICARUS on slf7 for c14:prof -- details available through the CI dashboard

🚨 For more details about the failed phase, check the build ICARUS phase logs

parent CI build details are available through the CI dashboard

[FNALbuild]

❌ CI build for SBND Failed at phase build SBND on slf7 for c14:prof -- details available through the CI dashboard

🚨 For more details about the failed phase, check the build SBND phase logs

parent CI build details are available through the CI dashboard

[FNALbuild]

❌ CI build for ICARUS Failed at phase ci_tests ICARUS on slf7 for e26:prof - ignored warnings for build -- details available through the CI dashboard

🚨 For more details about the failed phase, check the ci_tests ICARUS phase logs

parent CI build details are available through the CI dashboard

[FNALbuild]

⚠️ CI build for SBND Warning at phase ci_tests SBND on slf7 for e26:prof - ignored warnings for build -- details available through the CI dashboard

🚨 For more details about the warning phase, check the ci_tests SBND phase logs

parent CI build details are available through the CI dashboard