When using the `NewEntryPointWriter` we copy unmodified files over to the new
entry-point in addition to writing out the source files that are processed by ngcc.
But we were not copying over associated source-map files for these unmodified
source files, leading to warnings in downstream tooling.
Now we will also copy over source-maps that reside as siblings of unmodified
source files. We have to make sure that the sources of the source-map point
to the correct files, so we also update the `sourceRoot` property of the copied
source-map.
Fixes#40358
PR Close#40429