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/composite-fail-with-output/push.yml

@@ -0,0 +1,14 @@
+name: composite-fail-with-output
+on: push
+
+jobs:
+  test-for-output:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ./actions/composite-fail-with-output
+      id: composite-fail-with-output
+      continue-on-error: true
+    - run: |
+        echo ${{steps.composite-fail-with-output.outputs.customoutput}}
+        exit ${{steps.composite-fail-with-output.outputs.customoutput == 'my-customoutput-green' && '0' || '1'}}