Co-existence of `-opaque` and normal binaries in the same context

[rgrinberg]

To support fast builds, the dev profile uses -opaque to produce better rules for incremental compilation which produce slower binaries. The reliance on profiles to provide this feature is an unfortunate consequences of not wanting to setup more rules than necessary per context. Once we have proper support for rule streaming #5251, this limitation will not exist and we'll be able to have -opaque rules in the same profile.

Concretely, the new world would look like this:

• We'll have a new native-opaque mode that sets up fast rules as --dev does today.
• In the dev profile, these native-opaque artifacts will be considered best
• In the release profile, the native artifacts will be considered best.

[ejgallego]

Do the native and native-opaque rules produce the same targets?

[rgrinberg]

One would produce foo.native.exe and the other would produce foo.native.opaque.exe. best would select one of them to be foo.exe