Move core actions to a subpackage

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

tests/tasks/export/export.cue

@@ -2,21 +2,22 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
 	actions: test: {
-		image: dagger.#Pull & {
+		image: core.#Pull & {
 			source: "alpine:3.15"
 		}
 
-		export: dagger.#Export & {
+		export: core.#Export & {
 			input:  image.output
 			config: image.config
 			tag:    "example"
 		}
 
-		verify: dagger.#Exec & {
+		verify: core.#Exec & {
 			input: image.output
 			mounts: exported: {
 				contents: export.output