Normalize paths in manifest generation

[mitchdenny]

Currently we use \\ in the manifest, we should standardize on /.

[davidfowl]

@BrennanConroy can you take this one?

[BrennanConroy]

\ is a valid character in a directory/file name on Unix.

For prior-art, the Runtime made a breaking change to remove automatically converting \\ -> /. dotnet/runtime#78995

We could scope the conversion to only occur when creating the manifest on Windows and only when we call Path methods since that would always be a valid conversion.

[davidfowl]

We want the manifest to always have forward slashes.