Move core actions to a subpackage

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

tests/plan/hello-europa/main.cue

@@ -2,21 +2,21 @@ package main
 
 import (
 	"dagger.io/dagger"
-	// "alpha.dagger.io/os"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
 	actions: test: {
-		_image: dagger.#Pull & {
+		_image: core.#Pull & {
 			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
 		}
 
-		_exec: dagger.#Exec & {
+		_exec: core.#Exec & {
 			input: _image.output
 			args: ["sh", "-c", "echo -n Hello Europa > /out.txt"]
 		}
 
-		_verify: dagger.#ReadFile & {
+		_verify: core.#ReadFile & {
 			input: _exec.output
 			path:  "/out.txt"
 		} & {