geo backend should be in sf, not sp

[MichaelChirico]

Per comment here, sp is to be considered as deprecated for vector (i.e., non-raster) work:

https://twitter.com/edzerpebesma/status/1272928917700165633

[MichaelChirico]

Seems Edzer deleted his Twitter account. Anyway, this remains "urgent"... switching away from 'rgdal' was more painful because everything is still 'sp'-based.