This is a corner case, and converting them is what
was expected in G3. This also fits the fact that
we already convert package paths into relative paths.
PR Close#18912
Windows paths have back slashes, but TypeScript expects to always have forward slashes.
In other places where this call happens (like `src/compiler_host.ts`) the same fix is present.
PR Close#18784
This also allows to customize the filePaths in `.ngsummary.json` file
via the new methods `toSummaryFileName` and `fromSummaryFileName`
on the `CompilerHost`.
