Fixed difference of the $dist->{pathname} from the tarballs URI.

I found `--save-dists` problem, if we want save from tarballs URI are:

```bash
$ cpanm --save-dists tmp http://cpan.metacpan.org/authors/id/M/MI/MIYAGAWA/App-cpanminus-1.5021.tar.gz
$ find tmp -name "*tar.gz"
tmp/authors/id/http:/cpan.metacpan.org/authors/id/M/MI/MIYAGAWA/App-cpanminus-1.5021.tar.gz
```

Looks like bad saved paths. But I fixed it. Please check it.

Tanks,