Move core actions to a subpackage

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

tests/tasks/copy/copy_exec.cue

@@ -2,15 +2,16 @@ 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
 			args: [
 				"sh", "-c",
@@ -20,7 +21,7 @@ dagger.#Plan & {
 			]
 		}
 		test: {
-			verify_file: dagger.#ReadFile & {
+			verify_file: core.#ReadFile & {
 				input: exec.output
 				path:  "/output.txt"
 			} & {
@@ -28,14 +29,14 @@ dagger.#Plan & {
 				contents: "hello world"
 			}
 
-			copy: dagger.#Copy & {
+			copy: core.#Copy & {
 				input:    image.output
 				contents: exec.output
 				source:   "/output.txt"
 				dest:     "/output.txt"
 			}
 
-			verify_copy: dagger.#ReadFile & {
+			verify_copy: core.#ReadFile & {
 				input: copy.output
 				path:  "/output.txt"
 			} & {