Package pkg renamed into internal vol.2

[piotrkowalczuk]

New attempt to move files located in pkg directory into internal. A completely fresh start as it was quite impossible to rebase the branch after so many changes. Plus I don't have permission to origin anymore.

I encountered an issue:

internal/admission/webhook.go:49:2: unknown import path "github.com/travelaudience/aerospike-operator/internal/client/clientset/versioned": cannot find module providing package github.com/travelaudience/aerospike-operator/internal/clien
t/clientset/versioned
cmd/operator/main.go:42:2: unknown import path "github.com/travelaudience/aerospike-operator/internal/client/clientset/versioned/scheme": cannot find module providing package github.com/travelaudience/aerospike-operator/internal/client/
clientset/versioned/scheme
cmd/operator/main.go:43:2: unknown import path "github.com/travelaudience/aerospike-operator/internal/client/informers/externalversions": cannot find module providing package github.com/travelaudience/aerospike-operator/internal/client/
informers/externalversions
internal/backuprestore/handler.go:35:2: unknown import path "github.com/travelaudience/aerospike-operator/internal/client/listers/aerospike/v1alpha2": cannot find module providing package github.com/travelaudience/aerospike-operator/int
ernal/client/listers/aerospike/v1alpha2

I'm not able to find github.com/travelaudience/aerospike-operator/pkg/client package on master.

cc @pires

It is related to #1.