Broken symlinks behavior

https://pypi.org/project/beautifulsoup4/4.8.1/#files https://files.pythonhosted.org/packages/86/cd/495c68f0536dcd25f016e006731ba7be72e072280305ec52590012c1e6f2/beautifulsoup4-4.8.1.tar.gz includes a symlink `NEWS.txt` -> `CHANGELOG` and CHANGELOG doesnt appear in the archive because it doesnt appear in https://bazaar.launchpad.net/~leonardr/beautifulsoup/bs4/view/head:/MANIFEST.in (so also occurs in latter versions, and probably many earlier versions).

The resulting `FileNotFoundError` exception is logical, but it exposes the cache to the caller and the exception doesnt really explain what happened. A quick fix would be to detect this and raise a subclass of `FileNotFoundError` with a better error message.

```py
f.read()
File "/home/jayvdb/projects/dephell/dephell_archive/dephell_archive/_stream.py", line 65, in read
with path.open(self.mode, encoding=self.encoding) as stream:
File "/usr/lib64/python3.7/pathlib.py", line 1186, in open
opener=self._opener)
File "/usr/lib64/python3.7/pathlib.py", line 1039, in _opener
return self._accessor.open(self, flags, mode)
FileNotFoundError: [Errno 2] No such file or directory: '/path/to/cache/beautifulsoup4-4.8.1/NEWS.txt'
```

`strict-rfc3339-0.7` has a symlink which isnt broken, and it extracts fine into the cache and `.read()` returns the content of the linked file, which again is quite reasonable.

After https://github.com/dephell/dephell_archive/pull/2 , we should also add symlink related methods so the caller can detect & negotiate these symlinks like they would handle them if occurring on disk.