We currently insist that the FLINT library is built with its NTL interface.

This interface consists of just a few functions that could easily be provided instead by a small library separate from FLINT, or included in sagelib. https://github.com/wbhart/flint2/blob/trunk/interfaces/NTL-interface.cpp

This could help with modularization. Also the use of NTL by Singular is minimal.

flintlib/flint#1039

CC: @kiwifb @antonio-rojas

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/32908