Avoid non-API calls to `Rf_findVar()` and `Rf_findVarInFrame()`

[krlmlr]

Replace with equivalent idioms. If code generation is involved, adapt the code generator.

Found non-API calls to R: ‘Rf_findVar’, ‘Rf_findVarInFrame’

[krlmlr]

@schochastics @maelle: We'd like to push a fix to CRAN asap when it's good. Planning to do a patch release with just this feature. Just FYI, I can handle this, one or two extra pairs of eyes don't hurt.

[maelle]

@krlmlr Claude and @schochastics made #2597 actually

[krlmlr]

Missed that too, sorry.

[krlmlr]

Uh... we need to wait at least until tomorrow to see good results on GHA because R_getRegisteredNamespace() is so new. Reopening for now.