all-packages.nix: Provide a shorter way to specify packages

There are zillions of lines of the form

  foo = callPackage ../bla/foo { };

in all-packages.nix. To get rid of this verbosity, you can now list
such packages in pkgs/auto-packages.nix. This is just a list of
package file names, e.g.

  development/libraries/libogg
  development/libraries/libvorbis
  tools/archivers/gnutar

If the package needs non-default function arguments, or if its
intended attribute name is different from its file name, then you
cannot put it in auto-packages.nix and instead need to specify it in
all-packages.nix.

If Nix had a glob function (https://github.com/NixOS/nix/pull/235), we
could even get rid of auto-packages.nix and have package expressions
be discovered automatically. However, that might not be desirable
because of the need to traverse the file system to find packages we
may not even use.