ensure original matcher source is free of mutations (#47980)
In a previous PR (https://github.com/vercel/next.js/pull/46753), the
`originalSource` was saved to be passed along through the build process.
This was done a bit too late in the flow, which made `originalSource`
include some transformations.
Because `originalSource` is used for display purposes, we need it
unmodified. This PR preserves `originalSource`.
