Refine template structure for better development and testing

[ben-polinsky]

We've structured our new degit templates within our repo so they're tough to develop and test. The primary issue is - are the templates part of the monorepo, depending on other packages; or are they dumb templates, ready to publish. While we decided on the latter, it's not a great way to keep them in working shape.

To test our templates against other packages in the monorepo, we have to change their dep(s) to workspace: * in package json (and back before committing). We also will have to manually bump its dependencies when publishing.

I propose we create private packages which are template-apps for each of our templates. They are fully situated in the monorepo, easy to work on, and easy to test. When we publish our packages, we'll run a script which copies the package over to our template folder, replacing workspace:* versions dynamically with the version of that package about to be/just published.

[ben-polinsky]

In addition - I'd like to flatten the repo:

/templates
/test-apps
/packages