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/if-expressions/push.yml

@@ -0,0 +1,29 @@
+on: push
+jobs:
+  mytest:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      # - run: exit 1
+      - uses: ./
+        if: failure()
+      - run: echo Success
+        shell: bash
+      - run: echo Success
+        if: success()
+        shell: bash
+      - run: exit 1
+        shell: bash
+      - run: echo "Shouldn't run"
+        if: success()
+        shell: bash
+      - run: echo "Shouldn't run2"
+        shell: bash
+      - run: echo expected to run
+        if: failure()
+        shell: bash
+  next:
+    needs: mytest
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2