Move registry package into handler package

The goal is to free up the distribution/registry package to include common
registry types. This moves the webapp definitions out of the way to allow for
this change in the future.

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

registry/doc.go

@@ -0,0 +1,3 @@
+// Package registry is a placeholder package for registry interface
+// destinations and utilities.
+package registry