The only functionality of `standard_lib::std_crates` is adding `test` to
the requested crate list. `resolve_std` already unconditionally adds
`sysroot` to the list, which always includes `test` in dependency graph.
Therefore calling `std_crates` is redundant.

Instead of altering the original `-Zbuild-std` arg value,
this defers the timing of detereming which crate to build
to when Cargo generates std root units.

sysroot depends on `std`, `proc_macro`, and `test` [1],
which includes everything we need from the default set of crates.

Passing `sysroot` alone as spec should produce the same resolution.

[1]: https://github.com/rust-lang/rust/blob/acabb5248231987ae1f0c215208d1005a5db402d/library/sysroot/Cargo.toml#L7-L11

Will let `std:bool` to determine necessary deps for `-Zbuild-std`,
instead of erroring out.

In rust-lang#14183 Cargo starts bailing out if the `metadata.std`
field in a target spec JSON is set to `false`.
This is problematic because std for some targets are actually buildable
even they've declared as std-unsupported.

This patch removes the hard error, and instead determines the required
root crates by the `metadata.std` field. That is to say, if a target
is explicitly declared as `std: false`, `-Zbuild-std` will build `core`
and `compiler-builtins` only, no `std` will be built.

This patch doesn't change the behavior of `-Zbuild-std` with explicit
crates set. For example `-Zbuild-std=std` will force building `std`.

See Zulip discussion:
https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/help.20debugging.20a.20docs.2Ers.20issue.20with.20a.20new.20cargo.20error