This makes it possible to set values for the invocation via env
variables in the pod template, in particular to use PROXY.

This has required the introduction of pkg/interfaces, defining an
interface for Service Event generation, so that the Varnish controller
can hold a reference to IngressController, without introducing
import cycles for pkg/varnish and pkg/controller.

This has brought in a refactoring of controller/worker, introducing
the SyncObj to pass into the queue to workers. Now workers don't
have to guess at whether they're adding, updating or deleting.

And remove some dead code while we're at it.

Some elements were changed to be unexported, since they didn't need
to be exported or publicly documented.

cmd/ now only contains main.go.

And update docs for self-sharding as configured by VarnishConfig.

This adds the client API for the Custom Resource in the group
ingress.varnish-cache.org, with generated code for the API.

Also adds the pkg/ path to the project, and moves the Makefile up
to the root directory, so that code builds in pkg/ and cmd/.

Enables out-of-cluster runs.

This commit includes a comprehensive refactoring of the
controller:

- ResourceEventHandlers dispatch work to a workqueue for
  a worker that in turn dispatches work to a separate
  queue for each namespace. For each namespace, there
  is a worker that syncs the various resources.

- The controller uses a SharedInformer, with Informers
  and Listers for each resource type. The Listers are
  handed off to the namespace-specific workers.

- Separate source files contain code for syncing
  Endpoints, Ingress, Service and Secret.

- Added the controller CLI options:
  -namespace: for single-namespace deployments
  -templatedir: to locate the templates for VCL

- The director for templates is given either by the
  templatedir CLI option or the TEMPLATE_DIR env
  variable. This makes it possible to run the
  controller from the command-line, and run tests
  from any working directory.

- The Ingress annotation
  ingress.varnish-cache.org/varnish-svc identifies
  the Service name of the Varnish Service intended
  to implement it. If the annotation is absent,
  then the controller looks for exactly one Service
  in the namespace with the label "app:varnish-ingress".
  If none is found, the Ingress is rejected.
  (This is not yet documented.)

- Docs and examples updated

- Example for single-namespace deployment added.

When go.mod & go.sum are unchanged in the layer, no downloads are
necessary. This greatly saves time for the image builds.