BUILD: add dependency rule to prevent st2 deps in install script

packaging/BUILD.venv

@@ -1,3 +1,21 @@
+# rules on what packaging can depend on
+__dependencies_rules__(
+    (
+        # All python sources in this directory
+        "<python_*>[/*]",
+        (
+            # may depend on 3rd party dependencies,
+            "//reqs#*",
+            # and on anything in this diretory,
+            "/**",
+            # but nothing else (eg not st2common, st2*, runners, ...).
+            "!*",
+        ),
+    ),
+    # other targets (not python in this directory) may depend on anything.
+    ("*", "*"),
+)
+
 python_sources()
 
 # We use st2-py*.pex to quickly build a venv (like /opt/stackstorm/st2)