refactor: move act under internal

The runner is an application, not a library. `act/model` and
`act/exprparser` were the last packages anything outside this repository
consumed and they now live in actionslib, so nothing needs the rest of
`act` to be importable, and keeping it importable invites the coupling
that was just removed.

Import paths only, the files are unchanged.

Assisted-by: Codet:GPT-5.1-Codex

internal/act/runner/testdata/local-action-dockerfile/push.yml

@@ -0,0 +1,26 @@
+name: local-action-dockerfile
+on: push
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ./actions/docker-local
+      id: dockerlocal
+      with:
+        who-to-greet: 'Mona the Octocat'
+    - run: '[[ "${{ env.SOMEVAR }}" == "Mona the Octocat" ]]'
+    - run: '[ "${SOMEVAR}" = "Not Mona" ] || exit 1'
+      env:
+        SOMEVAR: 'Not Mona'
+    - run: '[[ "${{ steps.dockerlocal.outputs.whoami }}" == "Mona the Octocat" ]]'
+    # Test if overriding args doesn't leak inputs
+    - uses: ./actions/docker-local-noargs
+      with:
+        args: ${{format('"{0}"', 'Mona is not the Octocat') }}
+        who-to-greet: 'Mona the Octocat'
+    - run: '[[ "${{ env.SOMEVAR }}" == "Mona is not the Octocat" ]]'