Facets now don't fail if faceting vars are COL, ROW, etc.

[eliocamp]

This will close #4138.

Works around the issue of duplicated column names by explicitly selecting either data columns or layout columns in the layout data.frame when needed. Added tests that pass.

reprex :)

library(ggplot2)
mtcars$PANEL <- mtcars$cyl


ggplot(mtcars, aes(hp, mpg)) +
  geom_point() +
  facet_grid(cyl ~  am) 

ggplot(mtcars, aes(hp, mpg)) +
  geom_point() +
  facet_grid(PANEL ~  am) 

^{Created on 2021-01-12 by the reprex package (v0.3.0)}