Add functionality to `cargo package` to list the files that are/would be included in a package

[huonw]

E.g.

cargo package --help
...
  -l, --list              Print the files that would be included in a package without creating one.

[lifthrasiir]

This may be a possible workaround for #849.

[alexcrichton]

Currently cargo package -v will show the list of files that are packaged as they're packaged, but having a read-only version would also probably be good!