build-std: Don't treat std like a "local" package.

[ehuss]

This changes it so that build-std will not treat the std crates like a "local" package. This has the following changes:

• std does not build with incremental. This generally shouldn't be needed, and incremental has various bugs with std crates.
• Cargo's dep-info fingerprint tracking will not track the std crate sources, these are tracked via other means.
• Cargo's .d dep-info file does not include std crate sources.
• Lints are capped to "allow" for std crates, and warnings are not shown by default.

Closes rust-lang/wg-cargo-std-aware#44
Closes rust-lang/wg-cargo-std-aware#55

[rust-highfive]

r? @Eh2406

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[ehuss]

I heavily considered the approach suggested in rust-lang/wg-cargo-std-aware#44 (comment) to add a field to Package, but that seemed to be potentially much more difficult to wire up. Packages tend to prefer to be immutable, and this would require teaching Workspace to know it is the std workspace. I'd like to avoid having is_std infect the entire codebase. The downside to not doing that is that the calls to get_profile() need to pass in an explicitly computed "is_local" value. Willing to reconsider, but I think it will end up being a little more complex.

Also, I'm totally up for using a different phrase than "local", open to suggestions!

[alexcrichton]

@bors: r+

Nah this looks good to me. Thanks for this!

[bors]

📌 Commit b6a4b07 has been approved by alexcrichton

[bors]

⌛ Testing commit b6a4b07 with merge ceef92d...

[bors]

☀️ Test successful - checks-azure
Approved by: alexcrichton
Pushing ceef92d to master...