Encapsulate or export `{wrap/grid}_as_facets_list()`.

[teunbrand]

For relatively straightforward facet extensions that borrow the grid/wrap layout structure (but don't directly re-use the facet_wrap()/facet_grid() constructors), it would be convenient to access the wrap_as_facets_list() and grid_as_facets_list() functions. It is notoriously inconvenient to copy over these functions because they depend on a cascade of internal functions that also would need to be copied.

This cascade can be visualised as follows:

flow::flow_view_deps(ggplot2:::wrap_as_facets_list)
flow::flow_view_deps(ggplot2:::grid_as_facets_list)

For grid_as_facets_list():

If these are not suitable for export to users, could we encapsulate them in the ggproto classes so that they are accessible to extension developers?

[teunbrand]

We can just wrap facet_wrap()/facet_grid() if we need these, there is no strict need to access these internals.

facets <- facet_grid(vars(foo), vars(bar))$params[c("rows", "cols")]
facets <- facet_wrap(vars(foo, bar))$params$facets