pkgid: add the -p flag as the output suggests

pkgid: add the -p flag as the output suggests

When you use this command and there are multiple packages that match
this spec, the help instructions tell you to rerun it with the -p
argument which was previously not present. Since the code paths are
shared with 'cargo build' and 'cargo update', this adds the -p argument
to 'cargo pkgid' to make things more consistent.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>