Convert Windows paths as well when reading sourcemap (#86723)
Fixes this warning on Windows:
`C:\Users\IEUser\Desktop \next-turbopack-issue-repro\.next\dev|server\chunks\ssr\[turbopack]_runtime.js: Invalid source map. Only conformant source maps can be used to find the original code. Cause: Error:
sourceMapURL could not be parsed`
Because `pathToFileURL` was only called for absolute Unix paths, not Windows paths.
