If a user does e.g.
impl From<Bar> for foo::Foo
and get a compilation error about that `From<Bar>` is not implemented
for `Foo`, check if multiple versions of the crate with `Foo` is present
in the dependency graph. If so, give a hint about it.
I encountered this case in the wild and didn't realize I had multiple
versions of a crate in my dependency graph. So I was a bit confused at
first. This fix will make life easier for others.
