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:

Rdatatable/data.table#3076

[coveralls]

Coverage decreased (-0.07%) to 87.488% when pulling fe76d4a on MichaelChirico:dt_import into ed75f69 on mllg:master.

[mllg]

Thanks for the PR. I'll try to release a new version within in the next 4 weeks.