Make wp_normalize_path() on Windows resolve drive letter for drive–relative paths

[Rarst]

Though rarely used, Windows allows to omit drive letter in file path to treat is as drive–relative.

This causes inconsistency where paths pointing to the same dir are not normalized to the same representation by wp_normalize_path():

<?php
var_dump( wp_normalize_path( 'C:\server\www\dev' ) );
// "C:/server/www/dev

var_dump( wp_normalize_path( '\server\www\dev' ) );
// /server/www/dev << same path, but mismatch after normalize

var_dump( wp_normalize_path( realpath( '\server\www\dev' ) ) );
// C:/server/www/dev << resolved drive letter before normalize

I think drive letter should be explicitly resolved as part of normalization for this case.

[Rarst]

One problem is that wp_normalize_path() doesn’t require path to be real, it normalizes path strings really. Introducing file_exists() check here feels somewhat unwanted.

Maybe we should introduce a new function, something like wp_normalize_real_path() that will both validate path as valid filesystem path and normalize it?