Enforce linter to not cross packages

[lahabana]

Description

While working on: #12982

I've noticed an import from test/ into pkg/ this should never happen even for tests there's a pkg/test package already designed for these kind of things.
Same thing for app/ or api/ they should have strong linter rules to provide good logic separation.

golangci-lint has ways to restrict imports. We should add this:

kuma/.golangci.yml

Lines 92 to 101 in b1c0b13

	depguard:
	rules:
	pkg-import:
	list-mode: lax # only disallow some imports
	files:
	- "!**/app/**/*.go"
	- "!**/pkg/api-server/gui_handler.go"
	deny:
	- pkg: "github.com/kumahq/kuma/app"
	desc: "pkg can't import app"