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/basic/push.yml

@@ -0,0 +1,40 @@
+name: basic
+on: push
+
+env:
+  TEST: value
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - run: '[[ "$(pwd)" == "${GITHUB_WORKSPACE}" ]]'
+      - run: echo ${{ env.TEST }} | grep value
+      - run: env
+      - uses: docker://node:24-bookworm-slim
+        with:
+          somekey: ${{ env.TEST }}
+          args: echo ${INPUT_SOMEKEY} | grep somevalue
+      - run: ls
+      - run: echo 'hello world'
+      - run: echo ${GITHUB_SHA} >> $(dirname "${GITHUB_WORKSPACE}")/sha.txt
+      - run: cat $(dirname "${GITHUB_WORKSPACE}")/sha.txt | grep ${GITHUB_SHA}
+  build:
+    runs-on: ubuntu-latest
+    needs: [check]
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ./actions/action1
+        with:
+          args: echo 'build'
+  test:
+    runs-on: ubuntu-latest
+    needs: [build]
+    steps:
+      - uses: docker://node:24-bookworm-slim
+        with:
+          args: env
+      - uses: docker://node:24-bookworm-slim
+        with:
+          entrypoint: /bin/echo
+          args: ${{github.event_name}}