Improved handling of drive-like paths under Posix

- if a Windows systems emulates a Posix system, it shall
  handle both the cases of a mapped-in real file, and a native
  file that has a drive-like name - this handling has been
  improved no lower chances of false drive detection
- see #558

pyfakefs/fake_filesystem.py

@@ -1695,11 +1695,20 @@ def _starts_with_drive_letter(self, file_path):
             the path starts with a drive letter.
         """
         colon = matching_string(file_path, ':')
-        # we also allow a drive letter if only the real fs is Windows
-        # to allow for real path names
-        return ((self.is_windows_fs or os.name == 'nt') and
-                len(file_path) >= 2 and
-                file_path[:1].isalpha and (file_path[1:2]) == colon)
+        if (len(file_path) >= 2 and
+                file_path[:1].isalpha and file_path[1:2] == colon):
+            if self.is_windows_fs:
+                return True
+            if os.name == 'nt':
+                # special case if we are emulating Posix under Windows
+                # check if the path exists because it has been mapped in
+                # this is not foolproof, but handles most cases
+                try:
+                    self.get_object_from_normpath(file_path)
+                    return True
+                except OSError:
+                    return False
+        return False
 
     def _starts_with_root_path(self, file_path):
         root_name = matching_string(file_path, self.root.name)

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -796,6 +796,16 @@ def test_macos(self):
         self.assertTrue(os.path.ismount('/'))
         self.assertFalse(os.path.ismount('//share/foo'))
 
+    def test_drivelike_path(self):
+        self.fs.os = OSType.LINUX
+        folder = Path('/test')
+        file_path = folder / 'C:/testfile'
+        file_path.parent.mkdir(parents=True)
+        file_path.touch()
+        # use str() to be Python 3.5 compatible
+        os.chdir(str(folder))
+        self.assertTrue(os.path.exists(str(file_path.relative_to(folder))))
+
 
 if __name__ == "__main__":
     unittest.main()