[TE-5843] Encode v2 major version in go.mod module path

[gchan]

Description

The go.mod in this repo declares module github.com/buildkite/test-engine-client at every v2.x tag (v2.0.0 through v2.5.0 and main). Go's module versioning rules require modules at v2+ to encode the major version in the module path, so v2.x tags should read module github.com/buildkite/test-engine-client/v2.

The current state makes v2.x releases unfetchable via go get and uninstallable as a go tool. The +incompatible escape hatch doesn't apply because that only works for modules without a go.mod file:

$ go get github.com/buildkite/test-engine-client@v2.5.0
go: ... invalid version: module contains a go.mod file, so module path must
    match major version ("github.com/buildkite/test-engine-client/v2")

$ go get github.com/buildkite/test-engine-client/v2@v2.5.0
go: ... invalid version: module contains a go.mod file, so module path must
    match major version ("github.com/buildkite/test-engine-client/v2")

Downstream Go projects can install bktec as a tool (via the tool ( ... ) directive in go.mod) only against v1.x. buildkite/agent is pinned to v1.6.0 for exactly this reason — that's the most recent v1.x release, and predates the plan subcommand (v2.0.0) and --collect-git-metadata (v2.4.0).

This PR:

1. Updates go.mod to module github.com/buildkite/test-engine-client/v2.
2. Rewrites the 56 internal import paths in *.go files from github.com/buildkite/test-engine-client/internal/... to github.com/buildkite/test-engine-client/v2/internal/....
3. Updates the version ldflags path in .goreleaser.yaml from .../internal/version.Version to .../v2/internal/version.Version so release builds continue to stamp internal/version.Version correctly.

Once a v2.x tag is cut on top of this commit, go get github.com/buildkite/test-engine-client/v2@vX.Y.Z resolves and downstream go tool setups work. Historical v2.0.0–v2.5.0 tags don't need to move; downstream consumers should pin to the new tag (e.g. v2.5.1) and update their import paths to include the /v2 suffix.

Context

Resolves TE-5843.

Surfaced in TE-5842 while trying to unify two bktec call sites in buildkite/agent:

• .buildkite/steps/tests.sh uses go tool test-engine-client against v1.6.0 from go.mod.
• .buildkite/steps/collect-commit-metadata.sh (landed via TE-5828, agent#3924) downloads the v2.5.0 binary from the GitHub release at step time.

Changes

Single commit:

• [TE-5843] Encode v2 major version in go.mod module path

Verification

Backed by the existing test suite. The internal import rewrite is mechanical (sed-driven) so the diff is large but uniform.

go build ./...
go vet ./...
go test ./...

go build and go vet are clean. go test is clean on every package except internal/runner, which has the same pre-existing local-env failures on main (missing cypress, gotestsum, and cucumber binaries) — not regressions from this change.

AI assisted.

Deployment

Low risk. No behavioural change — only the module identifier and import paths move. The compiled binary is byte-equivalent modulo the internal/version.Version ldflag string, which is rewritten to match the new path in the same change.

Unblocks

• TE-5842 — bump test-engine-client in buildkite/agent and unify both call sites on go tool once v2.5.1 (or later) is tagged on top of this commit.

Rollback

Yes. Revert the merge; downstream consumers haven't been migrated yet because no v2.x tag exists with the corrected path.

[pda]

@codex review

[pda]

🚀 assuming Codex review doesn't spot anything unexpected.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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