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ARROW-7962: [R][Dataset] Followup to "Consolidate Source and Dataset classes"

This commit was pushed to #6470 along with my "approval" review, but a force push overwrote it.

cc @bkietz

Closes #6502 from nealrichardson/dataset-source-followup and squashes the following commits:

c1c5e7510 <Neal Richardson> Some R edits

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>