It should be an error to have [patch] in Cargo.toml that's not in a workspace root

[Boscop]

It should be an error to have [patch] in Cargo.toml that's not in a workspace root because it has no effect and may lead people to think that they are patching when they are not.

[exrook]

I just spent some time running in to this issue wondering why my [patch] section had no effect. I see now that it is briefly mentioned in the documentation here, but it would be nice to at least have a warning that the section is being ignored. I'd be happy to submit a patch if someone could point me in the right direction to start from. :)

[alexcrichton]

Oh awesome @exrook! Currently workspace validation happens around here and I think we'd want to start off with a warning (to avoid breaking older projects), but I suspect the fix for this would go around there