io: give event packages a common prefix

Packages that provide support for external events such as pointer, key and
system are only the beginning. Future packages are expected for clipboard
access, drag and drop, gps positions and so on.

To keep the number of top-level packages under control, move such I/O packages
to the new `io` directory.

The `system` package name was the previous solution to keeping the number of
top-level packages under control: I named it `system` instead of the narrower
`profile` because I expected to put all the less common events into it, turning
`system` into a "package util" smell.

With `io`, package system can be renamed to `profile`.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gesture/gesture.go

@@ -12,10 +12,10 @@ package gesture
 import (
 	"math"
 
-	"gioui.org/ui"
 	"gioui.org/f32"
 	"gioui.org/internal/fling"
-	"gioui.org/pointer"
+	"gioui.org/io/pointer"
+	"gioui.org/ui"
 )
 
 // Click detects click gestures in the form