Deprecates `getPayloadHMR` and simplifies this pattern into a single
`import { getPayload } from 'payload'`.
We will still retain the exported `getPayloadHMR` but it now will throw
a deprecation warning with instructions for how to migrate.
Live preview e2e tests had no CSS when tested against prod.
For all our other tests, we have a separate test/app directory that
imports CSS. Otherwise, the root-level /app directory is used.
For live-preview, we currently always run against test/live-preview/app,
that has no CSS import.
This PR adds a new test/live-preview/prod/app directory that imports CSS
and is used when we run tests against prod.
In order for this to work, I had to make import map generation smarter
