distro: Use `mirror://' URLs.

distro: Use `mirror://' URLs.

* distro/packages/base.scm, distro/packages/bash.scm,
  distro/packages/compression.scm, distro/packages/gawk.scm,
  distro/packages/gperf.scm, distro/packages/guile.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/pth.scm, distro/packages/readline.scm,
  distro/packages/recutils.scm: Use `mirror://gnu' URLs.
* distro/packages/gnupg.scm: Use `mirror://gnupg' URLs.