Register Enhances S3 methods more selectively

[MichaelChirico]

Cribbing this from Rdatatable/data.table#6589 -- our current approach always tries requireNamespace(). IMO it's better to avoid that for the very-weak Enhances dependency if we can.

[MichaelChirico]

Actually, IIUC we can fully drop this section of .onLoad() in favor of a NAMESPACE directive, however, I'm not sure how to generate the directive with {roxygen2}. I'll return to this when I have more time.

[AshesITR]

Isn't there @rawNamespace for manual entries?

[MichaelChirico]

Right, I'm just not sure if it's supported directly or we need to resort to @rawNamespace.