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/actions/composite-fail-with-output/action.yml

@@ -0,0 +1,13 @@
+outputs:
+  customoutput:
+    value: my-customoutput-${{ steps.random-color-generator.outputs.SELECTED_COLOR }}
+runs:
+  using: composite
+  steps:
+  - name: Set selected color
+    run: echo '::set-output name=SELECTED_COLOR::green'
+    id: random-color-generator
+    shell: bash
+  - name: fail
+    run: exit 1
+    shell: bash