io/gpio: enforce the vanity import URL

Change-Id: I1dba15914f26c4120dea09411ac2817c1ad1c16d
Reviewed-on: https://go-review.googlesource.com/24217
Reviewed-by: Johan Euphrosine <proppy@google.com>
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
diff --git a/io/gpio/driver/driver.go b/io/gpio/driver/driver.go
index dbbefd8..f1fb087 100644
--- a/io/gpio/driver/driver.go
+++ b/io/gpio/driver/driver.go
@@ -4,7 +4,7 @@
 
 // Package driver contains interfaces that needs to be implemented by
 // various GPIO implementations.
-package driver
+package driver // import "golang.org/x/exp/io/gpio/driver"
 
 // Direction determines the direction of the pin. A pin could be
 // configured to be an input or an output.
diff --git a/io/gpio/gpio.go b/io/gpio/gpio.go
index 896fa8a..35ad518 100644
--- a/io/gpio/gpio.go
+++ b/io/gpio/gpio.go
@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package gpio allows users to communicate with GPIO pins.
-package gpio
+package gpio // import "golang.org/x/exp/io/gpio"
 
 import "golang.org/x/exp/io/gpio/driver"