The new `per-package-target` feature does not work with `-Zbuild-std`

Problem

I just tried to use the new per-package-target feature implemented in https://github.com/rust-lang/cargo/pull/9030 (tracking issue) together with -Zbuild-std, but it results in an error: -Zbuild-std requires --target.

Steps

1. Add a default-target or forced-target key in Cargo.toml.
2. Try to compile by running cargo build -Zbuild-std=core.

Possible Solution(s)

I think the issue is that the target check already happens here: https://github.com/rust-lang/cargo/blob/db741ac733f3744c2d7ee593d5a01b18e6158b69/src/cargo/ops/cargo_compile.rs#L394-L399

But the default-target/forced-target fields are only considered later in generate_targets: https://github.com/rust-lang/cargo/blob/db741ac733f3744c2d7ee593d5a01b18e6158b69/src/cargo/ops/cargo_compile.rs#L925-L943

The generate_targets function is invoked here: https://github.com/rust-lang/cargo/blob/db741ac733f3744c2d7ee593d5a01b18e6158b69/src/cargo/ops/cargo_compile.rs#L470-L488
Notes

Output of cargo version:

cargo 1.53.0-nightly (4369396ce 2021-04-27)
release: 1.53.0
commit-hash: 4369396ce7d270972955d876eaa4954bea56bcd9
commit-date: 2021-04-27

cc @Ekleog

Start in src/cargo/ops/cargo_compile.rs, comparing the early -Zbuild-std target check with the later generate_targets handling of default-target and forced-target. Reproduce the issue with Cargo.toml containing one of those keys and cargo build -Zbuild-std=core. Done means the command no longer reports that -Zbuild-std requires --target when the per-package target configuration supplies the target.