[cli][eject] Support projects with dynamic or missing configs

[EvanBacon]

Why

• expo-cli supports running and publishing projects without a config or with a dynamic .js config so eject should too.
• crna templates don't have configs but people may want to eject them immediately to use them as bare templates.

How

• Strip slashes from name android package name suggestion to be more resilient.
  • ex: I run expo eject in a project folder my-cool-app (which doesn't have a config or name), the suggestion would've been blank cuz com.bacon.my-cool-app is invalid. Now it'll recommend com.bacon.mycoolapp.
• When the config file doesn't exist, write the generated config to ./app.json
  • we don't need to account for custom config paths --config because those are validated beforehand.
• Remove code that attempts to read from ./app.json which fails when a project uses a dynamic app.config.js file.