merge pages and app overlays (#60899)
### What
Merges the package into the internal overlay.
### Detailed Changes
* Move `@next/react-dev-overlay` into next package and rename the import
paths.
* Fix the `getErrorSource` symbol issue, use `Symbol.for()` instead of
`Symbol`
* Extra `getErrorSource` into single lib as it's being used in a lot of
places, this will keep the same bundle size
Closes PACK-2261
Closes NEXT-1977
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Co-authored-by: Jiachi Liu <inbox@huozhi.im>
