Add an example of importing packages themselves.

Another way Scala's imports are flexible is that they can import
packages themselves, not just their non-package members. This is
only natural if you think of nested packages being contained in
their surrounding package. For example, here, we import
`java.util.regex`. This makes `regex` usable as a simple name.

To access the `Pattern` singleton object from the java.util.regex
package, you can just say, `regex.Pattern`.