chore: weaken swingset dependency on vat-data to dev-dependency

Swingset *tests* use `@agoric/vat-data`, but not `src/`, so this
changes swingset/package.json to list it in `devDependencies`, not
`dependencies`.

Meanwhile, `@agoric/vat-data` lists swingset in its `devDependencies`
for its own tests. This gives us a cycle of `devDependencies` but not
normal dependencies.

Code which imports vat-data must (generally) be running inside a vat,
which means the application that decided to put that code into a vat
probably depends upon swingset: it had to import
`makeSwingsetController` to create the housing. But the vat-data
-importing code doesn't know that, and doesn't need a direct
dependency on swingset, because it never imports anything from
swingset itself.

packages/SwingSet/package.json

@@ -21,6 +21,7 @@
     "lint:eslint": "eslint '**/*.js'"
   },
   "devDependencies": {
+    "@agoric/vat-data": "^0.1.0",
     "@endo/ses-ava": "^0.2.21",
     "@types/tmp": "^0.2.0",
     "ava": "^3.12.1",
@@ -34,7 +35,6 @@
     "@agoric/store": "^0.6.10",
     "@agoric/swing-store": "^0.6.5",
     "@agoric/xsnap": "^0.11.2",
-    "@agoric/vat-data": "^0.1.0",
     "@endo/base64": "^0.2.21",
     "@endo/bundle-source": "^2.1.1",
     "@endo/captp": "^2.0.3",