fix mod serialization

[EvanBacon]

mods weren't being skipped in post-plugin serialization. I added a test that doesn't work due to the nature of our requireString() code. Tests just run like this without the fix:

(node:16415) UnhandledPromiseRejectionWarning: TypeError: Cannot destructure property 'modRequest' of '_a' as it is undefined.
(node:16415) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled w
ith .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:16415) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
 PASS   @expo/config  src/__tests__/getConfig-e2e-test.ts