We now include the prelude in so many places, this simplifies how we can
present how `cargo-test-support` works.
Yes, this included some `use` clean ups but its already painful enough
walking through every test file, I didn't want to do it twice.
While this is noisy and hides other deprecations, I figured deprecations would
make it easier for people to discover what tasks remain and allow us to
divide and conquer this work rather than doing a heroic PR.
In theory, this will be short lived and we'll go back to seeing
deprecations in our tests.
Per discussion in https://github.com/rust-lang/cargo/issues/6790. The
--out-dir CLI option and out-dir config option are often confused with
the OUT_DIR environment variable, when the two serve very different
purposes (the former tells Cargo where to copy build artifacts to,
whereas the OUT_DIR environment variable is set *by* Cargo to tell
build scripts where to place their generated intermediate artifacts).
Renaming the option to something less confusing is a prerequisite to
stabilizing it.
