gnu: Move 'which' to (gnu packages base).

gnu: Move 'which' to (gnu packages base).

* gnu/packages/which.scm: Remove.
* gnu-system.am (GNU_SYSTEM_MODULES): Remove it.
* gnu/packages/base.scm (which): New variable.
* gnu/packages/aspell.scm, gnu/packages/audio.scm,
  gnu/packages/autogen.scm, gnu/packages/cdrom.scm,
  gnu/packages/compression.scm, gnu/packages/datamash.scm,
  gnu/packages/games.scm, gnu/packages/glib.scm,
  gnu/packages/gnu-pw-mgr.scm, gnu/packages/gnutls.scm,
  gnu/packages/gps.scm, gnu/packages/guile.scm,
  gnu/packages/java.scm, gnu/packages/libevent.scm,
  gnu/packages/linux.scm, gnu/packages/lisp.scm,
  gnu/packages/node.scm, gnu/packages/ntp.scm,
  gnu/packages/orpheus.scm, gnu/packages/pciutils.scm,
  gnu/packages/python.scm, gnu/packages/qt.scm,
  gnu/packages/rsync.scm, gnu/packages/sawfish.scm,
  gnu/packages/scheme.scm, gnu/packages/ssh.scm,
  gnu/packages/statistics.scm, gnu/packages/texlive.scm,
  gnu/packages/wdiff.scm: Adjust accordingly.