Turbopack: Fix server action sourcemaps (#69884)
Closes #69861
Closes PACK-3240
The sourcemaps of server actions were invalid, the listed source file
was the file `AssetIdent` like
`turbopack://[project]/bench/basic-app/app/actions.ts [app-client]
(ecmascript, action)` instead of only the source file path.
This broke the sourcemap mapping logic of the error overlay.
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Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
