Auto merge of #5188 - alexcrichton:urgh-again, r=matklad

Add a synthetic dependency on num-traits

Right now the rust-lang/rust integration is compiling Cargo twice on dist
builds, once for Cargo and once for the RLS. This is due to a dependency of
Cargo being recompiled with different features when used from the RLS or not.
For now paper over this problem with a synthetic dependency to prevent Cargo
from being compiled twice.