Bugs in links_from_content

In the Dynamic Function [links_from_content](https://sphinx-needs.readthedocs.io/en/latest/dynamic_functions.html#links-from-content) are bugs:

First:
It is not detecting needpart, so ``:need:`id.part` `` is not working.

Secoundly, if we do have a content like:

```
content = """
:need:`foo <bar>`
:need:`foo3 <bar>`
"""
```

The output would be:
`['bar', 'bar']`
