Currently we are validating the `experimental.serverActions` flag when creating the actual entries for Server Actions, this causes two problems. One is that syntax errors caught at compilation time are still shown, even if you don't have this flag enabled. Another problem is we still traverse the client graph to collect these action modules even if the flag isn't enabled.
This PR moves that check to be happening at compilation time, which addresses the two above but also brings the extra benefit of showing the exact span and module trace that errors:
<img width="974" alt="CleanShot 2023-07-03 at 20 26 34@2x" src="https://github.com/vercel/next.js/assets/3676859/1676b1f6-e205-4963-bce4-5b515a698e9c">
If using Server Actions on the client layer without enabling the
`serverActions` feature, the build should error. Add a test case to
ensure #50199 works.
