bzltestutil: move os.Chdir call into new package

[jayconrod]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

Sorry for missing this the first time in #3679. The package init algorithm was a little different than I thought, and I imagined that Go 1.21 was already in use, so I didn't look into it deeply.

The problem was that even though "+initfirst/.../bzlutil" sorts lower than other packages, it imports "sync" and a number of other std packages that sort higher than "github.com/..." user packages. If a user package has no dependencies, it's eligible to be initialized before bzltestutil, even with the lower name.

I moved the init function with the os.Chdir call into a separate tiny package that only imports "os" and "runtime". It should get initialized much earlier.

Which issues(s) does this PR fix?

Fixes #3675

[jayconrod]

@fmeum I think this will fix the issue in #3679. I've verified at least that my company's affected vendored tests now pass when using 1.21.

[sluongng]

If you want to future proof this, you could add github.com/klauspost/compress into https://github.com/bazelbuild/rules_go/blob/master/tests/integration/popular_repos/popular_repos.py

I think that repo is quite relevant for the Bazel and Go ecosystem in general (we use it as well).

[jayconrod]

If you want to future proof this, you could add github.com/klauspost/compress into https://github.com/bazelbuild/rules_go/blob/master/tests/integration/popular_repos/popular_repos.py

I think that repo is quite relevant for the Bazel and Go ecosystem in general (we use it as well).

I tried, but it's not as easy as I hoped. Most of the tests in that repo read testdata files from a top-level testdata directory, so some hand-edited filegroups and data attributes would be needed: Gazelle / go_repository can't do it automatically. We've excluded tests in org_golang_x_tools for similar reasons.

Some tests are slow: zstd takes 99s on my machine.

There's also some desync between popular_repos.py and rules_go's dependencies.