Only add the pkg name if not part of the main name.
Only add the pkg name if not part of the main name.

This accomodates two scenarios.  Previous we were accepting as a main:

```json
{
  "name": "foo",
  "main": "main"
}
```

Now this also allows for a main of:

```json
{
  "name": "foo",
  "main": "foo/main"
}
```

This is more *correct* when using directories.lib. Fixes #69

stealjs/system-npm,stealjs/steal-npm,stealjs/steal-npm,stealjs/system-npm