Add a linter to remind us about our internal package

We should probably extend our discovery runner the next time we want to
use the discovery client.

.golangci.yaml

@@ -40,6 +40,9 @@ linters-settings:
           - pkg: github.com/crunchydata/postgres-operator/internal/testing/*
             desc: The "internal/testing" packages should be used only in tests.
 
+          - pkg: k8s.io/client-go/discovery
+            desc: Use the "internal/kubernetes" package instead.
+
       tests:
         files: ['$test']
         deny:
@@ -93,6 +96,11 @@ linters-settings:
 issues:
   exclude-generated: strict
   exclude-rules:
+    # This internal package is the one place we want to do API discovery.
+    - linters: [depguard]
+      path: internal/kubernetes/discovery.go
+      text: k8s.io/client-go/discovery
+
     # These value types have unmarshal methods.
     # https://github.com/raeperd/recvcheck/issues/7
     - linters: [recvcheck]