Replace os.PathSeparator as needed in UEFI fs.FS implementation.

[abarisani]

Currently the UEFI implementation of Go fs.FS interface expects callers to replace forward slashes with back slashes.

Rather it should satisfy fs.ValidPath requirement and only honor forward slashes.

For any other os access os.PathSeparator should be honored as needed.

[abarisani]

Solving this will aid cleaner integration of #6 which should always assume fs.FS requirements when creating paths for fs.FS and os.PathSeparator otherwise.

[abarisani]

This should be fixed in eafac4d, please merge development in #6 to improve the PR based on this bug fix.

Please close this issue once the fix is validated.