cmake: replace include(FindFFTW.cmake) with find_dependency(FFTW) in installed config

[Copilot]

Downstream consumers of the installed package get a CMake developer warning because Waves2AMRConfig.cmake directly include()s FindFFTW.cmake, causing find_package_handle_standard_args(FFTW …) to fire while CMAKE_FIND_PACKAGE_NAME is still Waves2AMR — a name mismatch CMake flags as a bug.

Changes

• cmake/Waves2AMRConfig.cmake.in: replace the include(…/FindFFTW.cmake) workaround with find_dependency(FFTW). The CMAKE_MODULE_PATH prepend (already present on line 3) makes FindFFTW.cmake discoverable, so find_dependency sets the correct CMAKE_FIND_PACKAGE_NAME=FFTW context before invoking the module.
• Remove stale comments explaining why find_dependency(FFTW) "doesn't work" — it does, once CMAKE_MODULE_PATH is set first.

# Before
#This works upstream
include("${PACKAGE_PREFIX_DIR}/@CMAKE_INSTALL_DATADIR@/cmake/Modules/FindFFTW.cmake")
#This does not and it feels like a bug because it doesn't invoke FindFFTW.cmake at all
#find_dependency(FFTW)

# After
find_dependency(FFTW)