fix(cmake): use dependency targets for build-tree consumers

[liamnwhite1]

Summary

Fix geometry-central's CMake target interface so build-tree consumers can override dependency targets while preserving the install/package behavior added for Nix.

What Changed

• Replaced hardcoded dependency include paths with target-based links.
• Linked Eigen3::Eigen and happly publicly for build-tree consumers.
• Linked nanoflann and nanort privately for build-tree consumers.
• Kept dependency target links scoped to BUILD_INTERFACE so the installed export remains clean.

Why

This addresses concerns raised on #190 about #238 making add_subdirectory() consumers use geometry-central's vendored dependency paths directly. Consumers can now provide their own compatible Eigen3::Eigen, happly, nanoflann, or nanort targets.

The installed package path from #238 is preserved: installed consumers still use find_package(GeometryCentral CONFIG) and inherit Eigen3::Eigen through the generated package config.

I intentionally kept the build-tree include path absolute with ${CMAKE_CURRENT_SOURCE_DIR}/../include rather than using a relative $<BUILD_INTERFACE:../include>. CMake documents that relative paths are allowed inside INSTALL_INTERFACE, where they are interpreted relative to the install prefix, but relative paths should not be used inside BUILD_INTERFACE because they are not converted to absolute paths. See: https://cmake.org/cmake/help/latest/command/target_include_directories.html

Verification

• Configured geometry-central with SUITESPARSE=OFF
• Built geometry-central
• Installed geometry-central
• Verified downstream find_package(GeometryCentral REQUIRED CONFIG) smoke build
• Verified downstream add_subdirectory() smoke build with predeclared dependency targets