Revert the revert in #66049
It was erroring in pages api with importing `react-dom/server` as this
is disallowed in app but shouldn't be in pages. It's caused by we're
validating middleware layer as server components but edge pages api is
still bundled in the same layer, where we shouldn't apply the check.
* Separate the api in api layers, and while handling middleware
warnings, checking api layer as well
* No need to check layers while handling externals in edge compiler
* Found a bug that we shouldn't check if `config.transpilePackages` is
defined then we enable `externalDir`, removed that condition. It fails
the telemetry tests case build with code change from this PR.
Add more tests for pages dir and middleware
| | `react` condition | `react-dom/server` condition |
| ---- | ---- | ---- |
| middleware (edge) | react-server | not allowed, failed with dev/build
checks |
| pages/api edge | default condition | default condition |
| pages/api node | default condition | default condition |
1415609870