Don't say "Unpacking" if installing from directory

##### Before

```
$ pip install ~/dev/git-repos/pip
Unpacking /Users/marca/dev/git-repos/pip
...
```

##### After

```
$ pip install ~/dev/git-repos/pip
Processing directory /Users/marca/dev/git-repos/pip
...
```

Fixes: GH-2187

Cc: @dstufft, @tomprince