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-via-composite-dockerfile/action.yml

@@ -0,0 +1,39 @@
+inputs:
+  who-to-greet:
+    default: 'Mona the Octocat'
+runs:
+  using: composite
+  steps:
+  # Test if GITHUB_ACTION_PATH is set correctly before all steps
+  - run: stat $GITHUB_ACTION_PATH/push.yml
+    shell: bash
+  - run: stat $GITHUB_ACTION_PATH/action.yml
+    shell: bash
+  - run: '[[ "$GITHUB_ACTION_REPOSITORY" == "" ]] && [[ "$GITHUB_ACTION_REF" == "" ]]'
+    shell: bash
+  - uses: ./actions/docker-local
+    id: dockerlocal
+    with:
+      who-to-greet: ${{inputs.who-to-greet}}
+  - run: '[[ "${{ env.SOMEVAR }}" == "${{inputs.who-to-greet}}" ]]'
+    shell: bash
+  - run: '[ "${SOMEVAR}" = "Not Mona" ] || exit 1'
+    shell: bash
+    env:
+      SOMEVAR: 'Not Mona'
+  - run: '[[ "${{ steps.dockerlocal.outputs.whoami }}" == "${{inputs.who-to-greet}}" ]]'
+    shell: bash
+  # 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: ${{inputs.who-to-greet}}
+  - run: '[[ "${{ env.SOMEVAR }}" == "Mona is not the Octocat" ]]'
+    shell: bash
+  # Test if GITHUB_ACTION_PATH is set correctly after all steps
+  - run: stat $GITHUB_ACTION_PATH/push.yml
+    shell: bash
+  - run: stat $GITHUB_ACTION_PATH/action.yml
+    shell: bash
+  - run: '[[ "$GITHUB_ACTION_REPOSITORY" == "" ]] && [[ "$GITHUB_ACTION_REF" == "" ]]'
+    shell: bash

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

@@ -0,0 +1,9 @@
+name: local-action-dockerfile
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ./local-action-via-composite-dockerfile