go-cloud-debug-agent: restrict building to Linux

This binary uses the golang.org/x/debug package, which cannot build
except under Linux. Non-Linux users get an error when they go install
or go test from the repo root.

Adding a build constraint to the main binary file fixes the problem.

Also, add an import identifier for a package with a different import
path, so goimports doesn't mistakenly delete the import.

Change-Id: If2659af1f98b0255a2c8e60fb64a6e191f9cc48e
Reviewed-on: https://code-review.googlesource.com/8576
Reviewed-by: John Dethridge <[email protected]>

cmd/go-cloud-debug-agent/debuglet.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build linux
+
 package main
 
 import (
@@ -26,7 +28,7 @@ import (
 	"time"
 
 	"cloud.google.com/go/cmd/go-cloud-debug-agent/internal/breakpoints"
-	"cloud.google.com/go/cmd/go-cloud-debug-agent/internal/controller"
+	debuglet "cloud.google.com/go/cmd/go-cloud-debug-agent/internal/controller"
 	"cloud.google.com/go/cmd/go-cloud-debug-agent/internal/valuecollector"
 	"cloud.google.com/go/compute/metadata"
 	"golang.org/x/debug"