Fix filename linter in Windows

[lpardosixtosMs]

When parsing the file names in Windows, tools/lint/lib/collect_files.py produces strings contain backslashes that are later flagged as invalid characters in the file name. This change normalizes the strings to always use forward slashes.

[ptomato]

Seems like we should use the posixpath module, or else amend the tools/lint/lib/checks/filename.py check to handle Windows paths.