Move core actions to a subpackage

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

tests/tasks/pull/pull.cue

@@ -2,10 +2,11 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
-	actions: pull: dagger.#Pull & {
+	actions: pull: core.#Pull & {
 		source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
 	} & {
 		// assert result

tests/tasks/pull/pull_auth.cue

@@ -2,6 +2,7 @@ package main
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
@@ -12,12 +13,12 @@ dagger.#Plan & {
 	}
 
 	actions: {
-		sopsSecrets: dagger.#DecodeSecret & {
+		sopsSecrets: core.#DecodeSecret & {
 			format: "yaml"
 			input:  client.commands.sops.stdout
 		}
 
-		pull: dagger.#Pull & {
+		pull: core.#Pull & {
 			source: "daggerio/ci-test:private-pull@sha256:c74f1b1166784193ea6c8f9440263b9be6cae07dfe35e32a5df7a31358ac2060"
 			auth: {
 				username: "daggertest"