Add support for build-dir layout v2

[ranger-ross]

This PR adds logic to handle the new layout of Cargo's build-dir which changes the directory structure of Cargo internally.
(can be enabled with -Zbuild-dir-new-layout)
While this is still unstable we are not expecting any large changes to the structure at this point.

Resources

• Tracking issue Re-organize build-dir by package + hash, rather than artifact type rust-lang/cargo#15010
• Call for testing

[ranger-ross]

Sorry about the diffs in this closure, I don't really understand why rustfmt has decided it wants to change this.
The only real change is adding both regexes to the walk_target_dir fn call

walk_target_dir(cx, &build_script_v1_layout_re, &build_script_v2_layout_re, &target_dir)

[ranger-ross]

hiding the whitespace changes in GitHub makes reviewing this a bit easier

[taiki-e]

Thanks for the PR! It looks good aside from the fact that I can't tell if it's actually working since there's no test.

[ranger-ross]

I added a test. I didn't know exactly how the test harness works so let me know if I did something wrong.
I copied the real1 test and enabled the new layout using the Cargo unstable flag (-Zbuild-dir-new-layout) via env var.

Before committing I modified the workspace_root in tests/auxilary/mod.rs to use .keep(), then verified the tests are actually using the correct layout.