fix: consistent module graph IDs during transform

[wesdimiceli]

Description

I was noticing that certain modules weren't updating when saving a change to their dependencies, and traced it back to this recent addition:
https://github.com/vitejs/vite/blob/v2/packages/vite/src/node/plugins/importAnalysis.ts#L676-L679

What's happening is some modules are given a browser-safe URL with the /@id/ prefix before they're added to the module graph. Then the above code strips the prefix and the imports are sent back through the transform pipeline, which adds duplicate modules without the prefix. This has the effect of creating a discontinuous module graph.

I would expect it to look like this:

entry.js -> /@id/__x00__virtual:my-module -> dependency.js

But instead the graph looks like this:

entry.js -> /@id/__x00__virtual:my-module
virtual:my-module -> dependency.js

Thus any changes made to dependency.js don't properly trickle down when purging the module cache and I must restart the server to see the changes.

I've been running this fix in an internal project for a few weeks via a yarn patch. Wanted to contribute the fix and a failing test.

Additional context

The fix essentially moves the /@id/ unwrapping to a more-central location alongside a tiny change to re-use module IDs if they already exist in the graph.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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