[turbopack] Emit error issue when missing entry files (src/index.js, pages/, app/)

[wbinnssmith]

Currently missing the src/index.js entry results in an empty page, with no feedback in the terminal about the missing entry file. This can lead to confusion if running turbopack from the wrong cwd.

This:

• Introduces OptionContentSourceVc, which signals this case. A custom Error type for missing entries may be an opportunity for refinement, but this is in line with @sokra's suggestion below. When all (src, pages, app) main content sources return a wrapped None, this issue is emitted.
• Publicizes and emits a NextAppIssue, which was previously only used for one other issue in next-core.

It now causes the following message to be written to the terminal:

error - [next app] path/to/testapp/
  An issue occurred while preparing your Next.js app
  Missing Next.js entry file(s): src/index.js, pages/*.js, and app/**/page.js are missing.

This still results in an empty page however. We should work on surfacing issues that occur before an HMR connection is established.

Test Plan: Removed src/index.js, pages/*.js, and app/**/page.js in a test app and confirmed the above message was logged to the terminal.

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[wbinnssmith]

From @sokra: this should only happen if we're missing a src/index.js, pages, and app directory (the entries for each ContentSource). Consider returning an Result<Option<ContentSourceVc>> from each, and if all are missing, emit an issue that covers the case where all are missing.

[github-actions[bot]]

:warning: CI failed :warning:

The following steps have failed in CI:

• Rust tests
• Rust benchmark tests

See workflow summary for details