Move core actions to a subpackage

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

tests/tasks/exec/mount_tmp.cue

@@ -2,19 +2,20 @@ 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"
 		}
 
-		exec: dagger.#Exec & {
+		exec: core.#Exec & {
 			input: image.output
 			mounts: temp: {
 				dest:     "/temp"
-				contents: dagger.#TempDir
+				contents: core.#TempDir
 			}
 			args: [
 				"sh", "-c",
@@ -24,7 +25,7 @@ dagger.#Plan & {
 			]
 		}
 
-		verify: dagger.#Exec & {
+		verify: core.#Exec & {
 			input: exec.output
 			args: [
 				"sh", "-c",