Deprecate &mod.fun/arity capture with complex expressions as modules

[josevalim]

The following code:

&expr.fun/arity

Should only work if:

• expr is a variable or an atom at compile-time
• fun is an atom at compile-time
• arity is an integer at compile-time

If those conditions are not met and it is valid, we should deprecate it.

Originally posted by @josevalim in #14090 (comment)