Move core actions to a subpackage

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

tests/tasks/merge/merge.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",
@@ -20,31 +21,31 @@ dagger.#Plan & {
 			]
 		}
 
-		dir: dagger.#Mkdir & {
+		dir: core.#Mkdir & {
 			input: dagger.#Scratch
 			path:  "/dir"
 		}
 
-		dirfoo: dagger.#WriteFile & {
+		dirfoo: core.#WriteFile & {
 			input:    dir.output
 			path:     "/dir/foo"
 			contents: "foo"
 		}
 
-		dirfoo2: dagger.#WriteFile & {
+		dirfoo2: core.#WriteFile & {
 			input:    dir.output
 			path:     "/dir/foo"
 			contents: "foo2"
 		}
 
-		dirbar: dagger.#WriteFile & {
+		dirbar: core.#WriteFile & {
 			input:    dir.output
 			path:     "/dir/bar"
 			contents: "bar"
 		}
 
 		test: {
-			merge: dagger.#Merge & {
+			merge: core.#Merge & {
 				inputs: [
 					dir.output,
 					dirfoo.output,
@@ -54,19 +55,19 @@ dagger.#Plan & {
 				]
 			}
 
-			verify_merge_output: dagger.#ReadFile & {
+			verify_merge_output: core.#ReadFile & {
 				input: merge.output
 				path:  "/output.txt"
 			} & {
 				contents: "hello world"
 			}
-			verify_merge_dirbar: dagger.#ReadFile & {
+			verify_merge_dirbar: core.#ReadFile & {
 				input: merge.output
 				path:  "/dir/bar"
 			} & {
 				contents: "bar"
 			}
-			verify_merge_dirfoo: dagger.#ReadFile & {
+			verify_merge_dirfoo: core.#ReadFile & {
 				input: merge.output
 				path:  "/dir/foo"
 			} & {