Move registry package out of repo root

Since the repo is no longer just the registry, we are moving the registry web
application package out of the repo root into a sub-package. We may break down
the registry package further to separate webapp components and bring the client
package under it. This change accomplishes the task of freeing up the repo root
for a distribution-oriented package. A stub doc.go file is left in place to
declare intent.

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

doc.go

@@ -0,0 +1,7 @@
+// Package distribution will define the interfaces for the components of
+// docker distribution. The goal is to allow users to reliably package, ship
+// and store content related to docker images.
+//
+// This is currently a work in progress. More details are availalbe in the
+// README.md.
+package distribution