Add command to print the package directory

`carton package-directory` prints the directory to which packages are
installed (essentially just the value of `package-user-dir`).


```
$ carton package-directory
/Users/swiesner/Developer/Projects/flycheck/.carton/24.3.50.1/elpa
```


Mostly intended to aid the writing of Makefiles, which may want the
package directory as dependency of a target.

Take a look at the changes to `Makefile` to see the intended use.


- [x] Add Ecukes feature.  I'll probably need your help here, @rejeep