go/packages: avoid logging context canceled error

If the context is canceled for packages.Load, the following log message is
written numerous times (spammed) to stderr:

internal error: error "context canceled" (*errors.errorString) without position

Check if the error is context.Canceled and don't log it if so.

The error will still be recorded in lpkg.Errors.

Signed-off-by: Christian Stewart <christian@aperture.us>

go/packages/packages.go

@@ -921,7 +921,9 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 			})
 
 			// If you see this error message, please file a bug.
-			log.Printf("internal error: error %q (%T) without position", err, err)
+			if err != context.Canceled {
+				log.Printf("internal error: error %q (%T) without position", err, err)
+			}
 		}
 
 		lpkg.Errors = append(lpkg.Errors, errs...)