feat: create an alias from pkg to package

[maiste]

From the interviews conducted by @leostera it appears that, intuitively, people tend to write package instead of pkg. To respond to this, I have added a dune package command, with the same sub commands as dune pkg and a test to make sure we keep track of both.

It doesn't seem that Cmdliner provides a way to do aliases, so I had to introduce a new command.