Switch to R_getRegisteredNamespace

[eddelbuettel]

Closes #1468

R-devel (very recently) added R_getRegisteredNamespace() allowing a lookup with in namespace with requiring the R_NamespaceRegistry symbol which is retired.

This minimal PR adjusts accordingly in one spot, and protects the condition by also selecting on the SVN commit number as the (automated) R-devel build we use in CI has not yet caught up.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

Moving the conditional from 4.5.0 to 4.6.0 for Rf_findVarInFrame, won't it generate NOTEs for 4.5.0?

[eddelbuettel]

Not for checking Rcpp itself. That may have been a 'during development of 4.6.0' change too.

[eddelbuettel]

Rebasing now after your merge. And indeed while that now works for 4.5.3 and R-devel it bites with the older stuff. Will have to add another #if branch.

[eddelbuettel]

Behaves locally under R 4.4.3, R 4.5.3 and R 4.7.0 so into reverse dependency checking it goes, first updating the machine to current CRAN and then we'll roll for the 2 1/2 or whatever it takes days.