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/act-composite-env-test/action1/action.yml

@@ -0,0 +1,21 @@
+name: action1
+description: action1
+runs:
+  using: composite
+  steps:
+  - name: env.COMPOSITE_OVERRIDE != '1'
+    run: exit 1
+    if: env.COMPOSITE_OVERRIDE != '1'
+    shell: bash
+  - name: env.JOB != '1'
+    run: exit 1
+    if: env.JOB != '1'
+    shell: bash
+  - name: env.GLOBAL != '1'
+    run: exit 1
+    if: env.GLOBAL != '1'
+    shell: bash
+  - uses: ./act-composite-env-test/action2
+    env:
+      COMPOSITE_OVERRIDE: "2"
+      COMPOSITE: "1"

internal/act/runner/testdata/act-composite-env-test/action2/action.yml

@@ -0,0 +1,21 @@
+name: action2
+description: actions2
+runs:
+  using: composite
+  steps:
+  - name: env.COMPOSITE_OVERRIDE != '2'
+    run: exit 1
+    if: env.COMPOSITE_OVERRIDE != '2'
+    shell: bash
+  - name: env.COMPOSITE != '1'
+    run: exit 1
+    if: env.COMPOSITE != '1'
+    shell: bash
+  - name: env.JOB != '1'
+    run: exit 1
+    if: env.JOB != '1'
+    shell: bash
+  - name: env.GLOBAL != '1'
+    run: exit 1
+    if: env.GLOBAL != '1'
+    shell: bash

internal/act/runner/testdata/act-composite-env-test/push.yml

@@ -0,0 +1,13 @@
+on: push
+env:
+  GLOBAL: "1"
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    env:
+      JOB: "1"
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ./act-composite-env-test/action1
+      env:
+        COMPOSITE_OVERRIDE: "1"