fzf: fix invalid symlink on vim-plugins

The fzf vim plugin wasn't working because it was making a symlink to a
directory with the full source code. This directory isn't present
anymore since the commit e95f17e2720e67e2eabd59d7754c814d3e27a0b2 wich
removes it because it isn't so useful for the go packages.

I fixed it by manually copying the plugin/ directory into the out
derivation, which is the only part of the source that contains the vim
plugin.

pkgs/tools/misc/fzf/default.nix

···
     cp $src/bin/fzf-tmux $bin/bin
     mkdir -p $man/share/man
     cp -r $src/man/man1 $man/share/man
-    mkdir -p $out/share/vim-plugins
-    ln -s $out/share/go/src/github.com/junegunn/fzf $out/share/vim-plugins/${name}
+    mkdir -p $out/share/vim-plugins/${name}
+    cp -r $src/plugin $out/share/vim-plugins/${name}
 
     cp -R $src/shell $bin/share/fzf
     cat <<SCRIPT > $bin/bin/fzf-share