I'm not sure just where we issue warnings, but it's certainly true that doing a glob import across crates can carry a measure of risk, since as new versions of the crate get published, that glob import can bring in more and more things. This is mostly problematic if you have two glob imports, because they might wind up conflicting with one another (if you have only one glob import, those names automatically take lesser priority to other, explicit imports).
In any case, I could imagine adding some kind of warning for glob imports across crates. Clippy likely has one already. Still, I think we want to be somewhat careful with the things we warn about -- Rust already issues a lot of lints by default. I've found these are super helpful for finding bugs, but can be kind of annoying when I'm just trying to hack something up and get it to work (precisely the sort of time I might be inclined to use a glob import). I've always wanted to see if we can find some kind of way to help strike that balance a bit better. (Part of the problem, I suppose, is that the compiler builds with #[deny(warnings)], which makes it awfully persnickety.)