Move core actions to a subpackage

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

tests/plan/with/with.cue

@@ -2,18 +2,19 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
 	actions: {
 		params: foo: string
 
-		_image: dagger.#Pull & {
+		_image: core.#Pull & {
 			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
 		}
 
 		test: {
-			[string]: dagger.#Exec & {
+			[string]: core.#Exec & {
 				input: _image.output
 				env: FOO: string
 				args: [