find_scale() works with scales from packages that are loaded but not attached

[krlmlr]

Closes #4705.

[thomasp85]

What is the performance repercussions of searching all loaded namespaces? There are many standard aesthetics for which scales do not exist, because they are used as-is so find_scale() is called quite a lot where it will end up looking through all loaded namespaces

[krlmlr]

How many find_scale() are there in a usual plot? Should we avoid calling find_scale() for bare unclassed vectors?

Implementers can optimize by adding an "env" attribute to their scale_type() return value too.

[thomasp85]

There will be one for every aesthetic not provided with an explicit scale in the plot construction... conserving this path for non-atomic aesthetics is probably a worthwhile addition...

Think about this, if we add it we should consider adding this approach to check_subclass() so that stats, geoms, and position can be looked up by name even in packages that are only loaded