Split rust_library into smaller rules

[hlopko]

As socialized on https://groups.google.com/g/rules_rust/c/kGMg6haEF44, we're going to split rust_library into smaller rules. This is the tracking issue for the effort.

Steps:

• Submit defs.bzl with rules in the desired end state
• Regenerate docs
• Provide buildozer migration script
• Migrate all uses of @rules_rust//rust:rust.bzl in the repository to @rules_rust//rust:defs.bzl
• Update raze and other closely related projects
• Wait for the community to migrate
• Remove the deprecated rust.bzl file. Currently planned for 2021-10-26.

[hlopko]

Migration script I used to migrate rules_rust, kythe, and internal codebase. I apologize for poor engineering:

#!/usr/bin/env rust-script
//! ```cargo
//! [dependencies]
//! duct = "0.13.5"
//! anyhow = "1.0.44"
//! ```

use duct::cmd;
use std::collections::BTreeSet;
fn main() -> anyhow::Result<()> {
    let all_targets = cmd!("buildozer", "print label", "//...:*")
        .read()?
        .lines()
        .map(|l| l.trim().to_owned())
        .collect::<Vec<_>>();
    let all_packages = all_targets
        .iter()
        .map(|lbl| format!("{}:__pkg__", lbl.split(':').next().unwrap()))
        .collect::<BTreeSet<_>>();
    let all_targets_with_crate_type = cmd!("buildozer", "print label crate_type", "//...:*")
        .read()?
        .lines()
        .filter(|l| !l.contains("(missing)"))
        .map(|l| l.split_once(' ').unwrap())
        .map(|(a, b)| (a.to_string(), b.to_string()))
        .collect::<Vec<_>>();

    for (crate_type, rule) in [
        ("staticlib", "rust_static_library"),
        ("cdylib", "rust_shared_library"),
        ("proc-macro", "rust_proc_macro"),
    ] {
        let targets_of_type = all_targets_with_crate_type
            .iter()
            .filter(|(label, ctype)| ctype == crate_type)
            .map(|(label, _)| label)
            .collect::<Vec<_>>();
        cmd(
            "buildozer",
            vec![
                "-k".to_string(),
                format!("new_load @rules_rust//rust:defs.bzl {}", rule),
            ]
            .iter()
            .chain(targets_of_type.iter().cloned()),
        )
        .run()
        .ok();
        cmd(
            "buildozer",
            vec!["-k".to_string(), format!("set kind {}", rule)]
                .iter()
                .chain(targets_of_type),
        )
        .run()
        .ok();
    }
    cmd(
        "buildozer",
        vec!["-k".to_string(), "remove crate_type".to_string()]
            .iter()
            .chain(all_targets_with_crate_type.iter().map(|(label, _)| label)),
    )
    .run()
    .ok();
    println!("########################### Moving package to top");
    cmd(
        "buildozer",
        vec!["-k".to_string(), "fix movePackageToTop".to_string()]
            .into_iter()
            .chain(all_packages.iter().cloned()),
    )
    .run()
    .ok();

    println!("########################### Migrating from rust.bzl to defs.bzl");
    for rule in [
        "rust_library",
        "rust_binary",
        "rustfmt_test",
        "rust_clippy",
        "rust_test",
        "rust_doc",
        "rust_doc_test",
        "rust_analyzer",
    ] {
        cmd(
            "buildozer",
            vec![
                "-k".to_string(),
                format!("replace_load @rules_rust//rust:defs.bzl {}", rule),
            ]
            .into_iter()
            .chain(all_packages.iter().cloned()),
        )
        .run()
        .ok();
    }

    println!("########################### Removing unused loads");
    cmd(
        "buildozer",
        vec!["-k", "fix unusedLoads"]
            .into_iter()
            .chain(all_packages.iter().map(|a| a.as_str())),
    )
    .run()
    .ok();
    Ok(())
}