Seems interesting, but I don't know if it could be implemented in a simple manner. I think it would create quite a lot of confusion especially in the following situation: package A requires package B, package C requires a "modified" B, and package D requires both A and C. The conflict doesn't seem solvable, even with namespaces: B and B modified would live in the same namespace, unless the latter modifies also the namespace.
I suggested adding "interfaces" to packages. So A might require <B>v1, while C requires <B>v1-modified. At the very least, loading C would fail if the copy of B available on the user's machine is the original that does not provide <B>v1-modified.
OF course, it still requires that the modified B provide the semantics for the original B so that it provides <B>v1 seamlessly.
