Auto merge of #8003 - ehuss:proc-macro-index, r=alexcrichton

Add proc-macro to index, and new feature resolver.

This adds the "pm" field to the index so that Cargo can detect which packages contain a proc-macro without downloading the package.

The second commit builds on that to support proc-macros in the new "de-unification" of the new feature resolver. This prevents dependencies shared between proc-macros and other dependency kinds from having features unified.

cc #7915

diff --cc crates/cargo-test-support/src/registry.rs
Simple merge

diff --cc src/cargo/core/compiler/unit_dependencies.rs
Simple merge

diff --cc src/cargo/core/profiles.rs
Simple merge

diff --cc src/cargo/core/summary.rs
Simple merge

diff --cc src/cargo/ops/cargo_compile.rs
Simple merge

diff --cc src/cargo/sources/registry/index.rs
Simple merge

diff --cc src/cargo/sources/registry/mod.rs
Simple merge

diff --cc src/cargo/util/toml/mod.rs
Simple merge

diff --cc src/doc/src/reference/unstable.md
Simple merge

diff --cc tests/testsuite/profile_config.rs
Simple merge