No longer import sp by default

[olivroy]

The goal of this PR is to reduce the number of imports by removing the sp package eventually.

Now that sp imports sf and raster imports terra, it would be great if leaflet normalized objects to terra.

Before this PR, an sp object would get normalized inside leaflet, but since sp uses sf under the hood, it is better to just convert to sf in leaflet, so that it can use a common leaflet logic and leave the normalization to sf instead of having custom methods in leaflet.

I plan to replace raster usage by terra, but I will wait to see if there is appetite in leaflet for that kind of change.

Since raster and sp objects are more and more obsolete, it seems odd that leaflet continues to import these packages unconditionnally as they are used less and less nowadays.

[schloerke]

Given #928 and {sf} is the new version of {sp}, thank you!

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I plan to replace raster usage by terra, but I will wait to see if there is appetite in leaflet for that kind of change.

From https://github.com/rspatial/terra

terra replaces the raster package. The interfaces of terra and raster are similar, but terra is simpler, faster and can do more.

It's a welcome change!

[olivroy]

Thanks for the quick review @schloerke ! I addressed your comments

I reverted everything related to terra / raster as I plan to take this on in a future PR.

• I added more docs about this in the leaflet() docs
• To be clear, conversion from sp::Spatial* objects will generally succeed unless this is an invalid shape?
• I improved the warning to be signaled once and to include the srcref with rlang.
• I left the S3 methods but added comments to mark them as obsolete / unused

[schloerke]

Thank you @olivroy !

(I'll merge once the checks pass. Thank you for the separation of PRs regarding raster/terra)