Use cargo-config2 to load cargo config

[taiki-e]

This uses cargo-config2 to load Cargo configuration.

This brings the following improvements:

• More accurate cargo configuration loading and resolution.
• Fix installation failure on Rust 1.60 and 1.61 by removing dependency on target-spec. Fixes [bug] rustc version err when install from cargo or source #236
• Remove run-time dependency on unstable cargo config get. (Previously, this command was used in a form allowing failure, like rust-analyzer does.)

[jedbrown]

This commit appears to have broken the --doctests option. The previous commit worked.

$ cargo llvm-cov test --doctests
warning: --doctests option is unstable
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/usr/bin/sccache rustc - --crate-name ___ --print=file-names -C instrument-coverage --cfg=coverage --cfg=coverage_nightly --cfg=trybuild_no_target -Z unstable-options --persist-doctests /home/jed/src/cargo-llvm-cov/target/llvm-cov-target/doctestbins --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=cfg` (exit status: 1)
  --- stderr
  error: Unrecognized option: 'persist-doctests'

error: process didn't exit successfully: `/home/jed/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo test -Z doctest-in-workspace --manifest-path /home/jed/src/cargo-llvm-cov/Cargo.toml --target-dir /home/jed/src/cargo-llvm-cov/target/llvm-cov-target` (exit status: 101)
$ cargo --version
cargo 1.68.0-nightly (8c460b223 2023-01-04)

[taiki-e]

@jedbrown Thanks for the report and sorry for the breakage! I released the fix in 0.5.5 (7aace00).

[jedbrown]

Thanks!