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"

internal/act/container/testdata/docker-pull-options/config.json

@@ -0,0 +1,7 @@
+{
+  "auths": {
+          "https://index.docker.io/v1/": {
+                  "auth": "dXNlcm5hbWU6cGFzc3dvcmQK"
+          }
+  }
+}

internal/act/container/testdata/scratch/test.txt

@@ -0,0 +1 @@
+testfile

internal/act/container/testdata/utf8.env

@@ -0,0 +1,3 @@
+FOO=BAR
+HELLO=您好
+BAR=FOO

internal/act/container/testdata/valid.env

@@ -0,0 +1 @@
+ENV1=value1

internal/act/container/testdata/valid.label

@@ -0,0 +1 @@
+LABEL1=value1