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/container/testdata/Dockerfile

@@ -0,0 +1,5 @@
+FROM scratch
+ENV PATH="/this/path/does/not/exists/anywhere:/this/either"
+ENV SOME_RANDOM_VAR=""
+ENV ANOTHER_ONE="BUT_I_HAVE_VALUE"
+ENV CONFLICT_VAR="I_EXIST_ONLY_HERE"