Make `--packages` option default to `--packages=.packages` for format_coverage

[asartalo]

It took me a few days to figure out that I needed to add --packages=.packages to format_coverage. I kept seeing only coverage for my test files and when I enabled --report-on=lib, it was empty. Since most projects are single projects, I assume that would be the most common location.

Also I hope that there's more documentation on what those do options do. I didn't know the significance of '--packages=.packages' so I didn't think I needed to add it.

I can contribute code if you need help with these.

[liamappelbe]

We're deprecating this flag in favor of --package, which does have a reasonable default like you're suggesting.