In almost all cases, I think there is a single version of a given package that would work in practice, and I think it's confusing for users to have an application that has multiple versions of what they think of as the "same" package inside it. This may be less of an issue in Go because it's structurally typed, but in Dart you could get weird errors like "Expected a Foo but got a Foo" because those "Foo"s are actually from different versions of "foo". Requiring a single version avoids that.
I think this makes a strong case for not releasing major version upgrades that use the same package names. The very idea of two incompatible things having the same name should set off alarm bells. Instead of trying to make that work, we should be avoiding it.
In the absence of this principle, the Java ecosystem has developed a compensatory mechanism of packaging "shadowed" versions of their dependencies alongside their own code. This is an ugly hack to accomplish the same thing after the fact, so we are already incurring even more complexity than would be imposed by following this rule.