Flag to enable dev-dependencies in `cargo rustc`.

[cutsoy]

Describe the problem you are trying to solve
I'm building some custom tooling to enable writing UI tests for iOS and Android in Rust. In both cases, I can't use the built-in test harness because it can only generate binaries, and both iOS and Android don't accept regular / "unwrapped" binaries.

I'm currently using this to build a library with --cfg test:

cargo rustc --message-format=json --target=x86_64-apple-ios --lib -- \
    --cfg=test --crate-type=staticlib

This works, but it doesn't include the dev-dependencies.

Alternatively, I can include --profile=test, but this doesn't work very well:

• This will include the test harness unless harness = false in the manifest (for the lib target).
• The Cargo JSON output is incorrect when using --profile test on a library target: the library (lib___.a) is built, but the JSON output contains the binary path (___), not the library path.

Describe the solution you'd like
The easiest solution would be to have a profile that acts in every way like --profile dev, but includes the dev-dependencies (in commands::rustc and CompileFilter::need_dev_ops).

cargo rustc --message-format=json --target=x86_64-apple-ios --lib --profile=dev-dev -- \
    --cfg test --crate-type staticlib

For example, we might want:

- Some("dev") | None => CompileMode::Build,
+ Some("dev") | None => CompileMode::Build { dev_dependencies: false },
+ Some("dev-dev") => CompileMode::Build { dev_dependencies: true },

Alternatively, a separate flag could also work for me:

cargo rustc --message-format=json --target=x86_64-apple-ios --lib --dev -- \
    --cfg test --crate-type staticlib

Notes
I expected --profile=dev to already include the dev-dependencies (because of the prefix). This makes it a bit difficult to think of a better name for a profile that does include the dev-dependencies.