That's exactly what I meant in "In CL ... (b) using packages greatly reduces accidental shadowing." So I'm not sure what you are asking to be convinced. CL certainly does not prevent you from messing with package-private symbols, but usually a convention is enough to avoid disaster. I mean, I won't write (let ((your-package::private-symbol list)) (your-macro ...) ...) unless there's absolute need to do, such as emergency workaround of a bug in a third-party library.
On the other hand, one of the reasons Scheme avoids explicit module prefix is, I think, that it breaks abstraction. Suppose my-module defines my-macro and my-function. My-macro expands into a call to my-function. But I don't really want my module users to call my-function directly, so I only export my-macro and keep my-function private.
In your proposal, my-macro will expands into prefixed-symbol like this:
(define-macro (my-macro arg) `(my-module.my-function ,arg))
But... if we allow module-private function to be called with module prefix, we can't prevent ordinary code from calling my-function as well.
This is a language design choice. Some may not mind others to mess with module-private bindings; CLers certainly would not. But if you want to hide module-private symbols, and want to allow macros expand into them, how will you do that? One idea is to mark the output of macro expansion so that the compiler can distinguish whether the reference is from legitimate macro expansion or not... well, that's what hygienic macro is doing.
