There is another point worth discussing I think. When we want to fork a
package foo 1.0 because the version 2.0 is incompatible, do we:
1. duplicate the package foo as foo-1 and upgrade foo to the version 2.
Every reverse dependency that doesn't work with the version has to be
updated to use the new package foo-1.
2. fork and upgrade the package foo as foo-2. The package that need the
new version depend on foo-2 and the other remain unchanged.