Rewrite the CPAN generator to Perl

Also:

- It's now installable by doing "nix-env -i nix-generate-from-cpan".

- It maps dependencies to the correct attribute (e.g. HTML::HeadParser
  is mapped to HTMLParser).

- It automatically selects buildPerlPackage or buildPerlModule.

- It's documented in the manual.

maintainers/scripts/nix-generate-from-cpan.nix

@@ -0,0 +1,22 @@
+{ stdenv, makeWrapper, perl, perlPackages }:
+
+stdenv.mkDerivation {
+  name = "nix-generate-from-cpan-1";
+
+  buildInputs = [ makeWrapper perl perlPackages.YAML perlPackages.JSON ];
+
+  unpackPhase = "true";
+  buildPhase = "true";
+
+  installPhase =
+    ''
+      mkdir -p $out/bin
+      cp ${./nix-generate-from-cpan.pl} $out/bin/nix-generate-from-cpan
+      wrapProgram $out/bin/nix-generate-from-cpan --set PERL5LIB $PERL5LIB
+    '';
+
+  meta = {
+    maintainers = [ stdenv.lib.maintainers.eelco ];
+    description = "Utility to generate a Nix expression for a Perl package from CPAN";
+  };
+}