`cargo rustc` broken for tests in project with bins

[ehuss]

If you have a project with a binary and an integration test, then cargo rustc --test test1 -- -C opt-level=3 will fail with the following error:

error: extra arguments to `rustc` can only be passed to one target, consider filtering
the package by passing e.g. `--lib` or `--bin NAME` to specify a single target

The reason is that I changed it to add the implicit bin targets earlier (into generate_targets when it was previously in compute_deps) in order to avoid some code duplication.

I think it should be fine just to move it back to compute_deps, the code duplication was very minor.

[matklad]

This is a regression, right? So we might want to backport it perhaps?

[ehuss]

This is a regression, right? So we might want to backport it perhaps?

For 1.27, I think so. If you want, if/when the current PR is approved, I can post another for the backport.

[matklad]

I can post another for the backport.

Yep, let's do the backport dance!