FoundationEssentials: ensure that the path exists before iteration (#665)

When performing a `subpathsOfDirectory`, ensure that the path exists. On
the non-Windows path, `_FTSSequence` will perform the initial `statvfs`
during construction which obscured this requirement. We instead
explicitly inline this call in the implementation. This repairs further
test failures.

Author: compnerd

Sources/FoundationEssentials/FileManager/FileManager+Directories.swift

@@ -159,6 +159,13 @@ extension _FileManagerImpl {
 
     func subpathsOfDirectory(atPath path: String) throws -> [String] {
 #if os(Windows)
+        try path.withNTPathRepresentation {
+            var faAttributes: WIN32_FILE_ATTRIBUTE_DATA = .init()
+            guard GetFileAttributesExW($0, GetFileExInfoStandard, &faAttributes) else {
+                throw CocoaError.errorWithFilePath(path, win32: GetLastError(), reading: true)
+            }
+        }
+
         var results: [String] = []
         for item in _Win32DirectoryContentsSequence(path: path, appendSlashForDirectory: true) {
             results.append(item.fileName)