Uses requirement parser to normalize package name.

Hi @mitsuhiko,

I found this issue by `pipsi install honcho[export]`, and the output of `pipsi list` is:

```
$ pipsi list
...
  Package "honcho-export-":
    honcho
...
```

The name `honcho-export-` is strange.

I tried to use `pkg_resources.Requirement.parse` to normalize the package names. Then the version will be striped and the name will be "safe" still, and the package names with extra requirements like `honcho[export]` could be parsed correctly now.

Could you review it? Thank you.