local variables with name package result in a syntax error

We should either document this / make it a clearer error, or we could change it to be allowed. Allowing it would make some things more tricky perhaps.

```
$ mirah -e 'package = 1234;puts package'

DashE:1:

ERROR: expected dotted_name before ' = 1234;puts '
package = 1234;puts package
       ^
DashE parsed to nil
1 errors
```
