Move core actions to a subpackage

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

tests/tasks/scratch/scratch.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
 			mounts: fs: {
 				dest:     "/scratch"

tests/tasks/scratch/scratch_build_scratch.cue

@@ -2,10 +2,11 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
-	actions: build: dagger.#Build & {
+	actions: build: core.#Dockerfile & {
 		source: dagger.#Scratch
 		dockerfile: contents: "FROM scratch"
 		// Assert that output is dagger.#Scratch

tests/tasks/scratch/scratch_writefile.cue

@@ -2,17 +2,18 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
 	actions: {
-		write: dagger.#WriteFile & {
+		write: core.#WriteFile & {
 			input:       dagger.#Scratch
 			path:        "/testing"
 			contents:    "1,2,3"
 			permissions: 700
 		}
-		readfile: dagger.#ReadFile & {
+		readfile: core.#ReadFile & {
 			input: write.output
 			path:  "/testing"
 		} & {