Move core actions to a subpackage

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

docs/tests/core-concepts/plan/structure.cue.fragment

@@ -23,7 +23,7 @@ dagger.#Plan & {
       run: bash.#Run & {
          // ...
       }
-      contents: dagger.#Subdir & {
+      contents: core.#Subdir & {
         // ...
       }
     }