data.table Depends->Imports

[MichaelChirico]

Thanks for using data.table! We noticed you are Depending on us and recently decided to strongly discourage downstream packages from doing so -- we think Imports is always better.

This PR attempts to make the migration from Depends to Imports for data.table.

If you have a strong reason for preferring Depends, we'd love to hear it over at our issue tracker:

https://github.com/Rdatatable/data.table/issues/3076