fix: keep the module root a single package (tools.go -> go.mod tool directive)

go.mod

@@ -2,6 +2,14 @@ module github.com/databricks/databricks-sql-go
 
 go 1.25.0
 
+// gotestsum is a build-time test runner (see the Makefile's test/coverage
+// targets), not a library the driver imports. The tool directive (Go 1.24+)
+// keeps it in the module graph — the job the old //go:build tools file did —
+// without declaring a second package in the module root.
+//
+// See https://github.com/databricks/databricks-sql-go/issues/162.
+tool gotest.tools/gotestsum
+
 require (
 	github.com/apache/arrow/go/v12 v12.0.1
 	github.com/apache/thrift v0.23.0
@@ -12,7 +20,6 @@ require (
 	github.com/pierrec/lz4/v4 v4.1.15
 	github.com/stretchr/testify v1.8.1
 	golang.org/x/oauth2 v0.27.0
-	gotest.tools/gotestsum v1.8.2
 )
 
 require (
@@ -43,6 +50,7 @@ require (
 	golang.org/x/tools v0.6.0 // indirect
 	golang.org/x/xerrors v0.0.0-20220609144429-65e65417b02f // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
+	gotest.tools/gotestsum v1.8.2 // indirect
 )
 
 require (

tools_test.go

@@ -0,0 +1,30 @@
+package dbsql
+
+import (
+	"go/build"
+	"testing"
+)
+
+// TestModuleRootIsASinglePackage guards the fix for #162: the module root must
+// declare exactly one package no matter which build tags are in play.
+//
+// The build-time test runner used to be pinned by a `//go:build tools` file
+// declaring `package tools` next to `package dbsql`. Tooling that resolves a
+// directory's package without honoring build constraints (bazel/gazelle) — or
+// that deliberately enables the `tools` tag (golangci-lint) — then saw two
+// package names in one directory and failed to load the driver. gotestsum is
+// pinned by the go.mod `tool` directive instead, so nothing but `dbsql` lives
+// here.
+func TestModuleRootIsASinglePackage(t *testing.T) {
+	ctx := build.Default
+	// Any tag a consumer might enable must not conjure a second package; "tools"
+	// is the one that historically did.
+	ctx.BuildTags = append(ctx.BuildTags, "tools")
+
+	if _, err := ctx.ImportDir(".", 0); err != nil {
+		if _, multiple := err.(*build.MultiplePackageError); multiple {
+			t.Fatalf("module root declares more than one package: %v", err)
+		}
+		t.Fatalf("could not load the module root: %v", err)
+	}
+}