Move core actions to a subpackage

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

tests/tasks/exec/user.cue

@@ -2,20 +2,21 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
 	actions: {
-		image: dagger.#Pull & {
+		image: core.#Pull & {
 			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
 		}
 
-		addUser: dagger.#Exec & {
+		addUser: core.#Exec & {
 			input: image.output
 			args: ["adduser", "-D", "test"]
 		}
 		test: {
-			verifyUsername: dagger.#Exec & {
+			verifyUsername: core.#Exec & {
 				input: addUser.output
 				user:  "test"
 				args: [
@@ -26,7 +27,7 @@ dagger.#Plan & {
 				]
 			}
 
-			verifyUserID: dagger.#Exec & {
+			verifyUserID: core.#Exec & {
 				input: addUser.output
 				user:  "1000"
 				args: [