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/set-env-step-env-override/push.yml

@@ -0,0 +1,24 @@
+on: push
+jobs:
+  _:
+    runs-on: ubuntu-latest
+    env:
+      MY_ENV: test
+    steps:
+    - run: exit 1
+      if: env.MY_ENV != 'test'
+    - run: |
+        runs:
+          using: composite
+          steps:
+          - run: exit 1
+            shell: bash
+            if: env.MY_ENV != 'val'
+          - run: echo "MY_ENV=returnedenv" > $GITHUB_ENV
+            shell: bash
+      shell: cp {0} action.yml
+    - uses: ./
+      env:
+        MY_ENV: val
+    - run: exit 1
+      if: env.MY_ENV != 'returnedenv'