Check files improve path normalization

[inqrphl]

1. path normalization function cleans up the paths better, by switching to filesystem separator first.

normalized path is used for depth calculation. the depth is the count difference of filesystem separators

it is also used to check if an filesystem item is inside another filesystem item.

for windows, the paths could be specified using forward slashes like this: "C:/Windows". This meant that this block in normalizePath would run and append a backward slash making the path "C:/Windows" .

	if runtime.GOOS == "windows" {
		// C: -> C:\
		if len(path) == 2 && unicode.IsUpper(rune(path[0])) && path[1] == ':' {
			// Ensure drive letters always have a backslash: "C:" -> "C:\"
			return path + string(os.PathSeparator)
		}
		// "C:example" -> "C:\example".
		if len(path) >= 3 && unicode.IsUpper(rune(path[0])) && path[1] == ':' && path[2] != '\\' {
			winBasePath := path[:2] + string('\\') + path[2:]

			return winBasePath
		}
	}

Discovered files would look like "C:/Windows/test.txt" and would not be considered as under "C:/Windows", breaking the check.

convert forward slashes to backward slashes using filepath.FromSlash before that block.

2. For all platforms:
   Replace groups of separators into a single separator. This prevents a////b////c from having higher depth than a/b/c .

Fixing this issue as well, it did not come from a bug report but found while looking through the code.