bpo-45548: Fix out-of-tree and Debian builds (GH-29263)

Add Modules subdirs to SRCDIRS to generate directories for out-of-tree
object files.

Debian wants ncurses lib. Works on Fedora, too.

Debian also needs pkg-config to detect correct flags.

Remove more outdated comments. Makefile now tracks header dependencies
-lintl is injected by configure when needed. Build _dbm with
gdbm-compat.

Group some modules by purpose. socket, select, and mmap work on Windows,
too.