Better expose the fftpack interface

[rouson]

This commit

1. Exposes and clarifies fftpack's public interface (the fftpack module) by isolating it in the src/ subdirectory and moving all other src files to a new fftpack subdirectory.
2. Adjusts the CMake build system accordingly.

No adjustment to the fpm manifest is necessary. Both build systems still work.

[zoziha]

Thanks for sharing, looks good to me!

[zoziha]

This folder structure change breaks the makefile build, but I think we can easily build the fftpack via cmake, meson, and fpm, and the makefile doesn't seem to be necessary, and we can remove support for it, like fortran-lang/stdlib.

Previously we did not set a CI test for the makefile, and the relevant files included:

1. makefile in each folder;
2. the makefile description in the README.