context: remove definition of Context

Back in the before time, the best practices surrounding usage of Context
weren't quite worked out. We defined our own type to make usage easier.
As this packaged was used elsewhere, it make it more and more
challenging to integrate with the forked `Context` type. Now that it is
available in the standard library, we can just use that one directly.

To make usage more consistent, we now use `dcontext` when referring to
the distribution context package.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

context/trace.go

@@ -1,6 +1,7 @@
 package context
 
 import (
+	"context"
 	"runtime"
 	"time"
 
@@ -36,7 +37,7 @@ import (
 //
 // Notice that the function name is automatically resolved, along with the
 // package and a trace id is emitted that can be linked with parent ids.
-func WithTrace(ctx Context) (Context, func(format string, a ...interface{})) {
+func WithTrace(ctx context.Context) (context.Context, func(format string, a ...interface{})) {
 	if ctx == nil {
 		ctx = Background()
 	}
@@ -69,7 +70,7 @@ func WithTrace(ctx Context) (Context, func(format string, a ...interface{})) {
 // also provides fast lookup for the various attributes that are available on
 // the trace.
 type traced struct {
-	Context
+	context.Context
 	id     string
 	parent string
 	start  time.Time