The fmt test will cause clippy dev to be compiled and run. The clippy dev
dependencies are currently stored at `target/debug/deps` when this happens.
This location sometimes causes conflicts with ui tests which result in the
build error "thread 'compile_test' panicked at 'Found multiple rlibs for crate
`regex` ...".
This commit forces the clippy_dev dependencies to be stored under
`clippy_dev/target/` which seems to resolve this problem.
If you've been using `./util/dev` before, this now becomes `cargo dev`.
The key part of this change is found in `.cargo/config`.
This means one less shell script and a bit more cross-platform support
for contributors.
