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/path-handling/action.yml

@@ -0,0 +1,21 @@
+name: output action
+description: output action
+
+inputs:
+  input:
+    description: some input
+    required: false
+
+outputs:
+  job-output:
+    description: some output
+    value: ${{ steps.gen-out.outputs.step-output }}
+
+runs:
+  using: composite
+  steps:
+    - name: run step
+      id: gen-out
+      run: |
+        echo "::set-output name=step-output::"
+      shell: bash