distro: Split (distro packages base) into several files.

* distro/packages/base.scm (libsigsegv, gawk, perl, m4, gmp, mpfr, mpc,
  ncurses, readline, bash, libtool, libunistring, libffi, pkg-config,
  libgc): Move to modules of their own.
  (guile-1.8, guile-2.0): Move to...
* distro/packages/guile.scm: ... here.

* distro/packages/bash.scm, distro/packages/bdw-gc.scm,
  distro/packages/gawk.scm, distro/packages/libffi.scm,
  distro/packages/libsigsegv.scm, distro/packages/libtool.scm,
  distro/packages/libunistring.scm, distro/packages/m4.scm,
  distro/packages/multiprecision.scm, distro/packages/ncurses.scm,
  distro/packages/perl.scm, distro/packages/pkg-config.scm,
  distro/packages/readline.scm: New files.