Fix next/server api alias for ESM pkg (#61721)
### What & Why
We have a modularize imports config for `next/server` before, which will
transform the `next/server` imports to directly import from the actual
file, for instance: `import { NextRequest } from 'next/server'` will
become `import { NextRequest } from
'next/dist/server/web/exports/next-request'`, where the NextRequest is
exported as default export. This is fine in most case until you're using
a ESM pkg, then it will be resolved as `{ default: NextRequest }`
according to the spec. Since it's a ESM import to a CJS module in
`next/dist`.
Since we already have the ESM alias introduced in #59852 , this can
handle the case more properly.
### How
Remove the modularize imports config for `next/server`, use the ESM api
alias instead.
Migrate the cjs optimizer tests from middleware to a separate endpoint
`/cjs/server`. As now ESM imports for next/server are not going to get
tree-shaken in dev, but since we don't have image response there it's
still fine.
Closes NEXT-2376
Closes NEXT-2374
