Use beautifulsoup4 CSS selectors to simplify code and type checking

As the types-beautifulsoup4 package gets installed in the swh virtualenv
as it is a swh-scanner test dependency, some mypy errors were reported
related to beautifulsoup4 typing.

As the returned type for the find method of bs4 is the following union:
Tag | NavigableString | None, isinstance calls must be used to ensure
proper typing which is not great.

So prefer to use the select_one method instead where a simple None check
must be done to ensure typing is correct as it is returning Optional[Tag].
In a similar manner, replace use of find_all method by select method.

It also has the advantage to simplify the code.

swh/lister/nuget/lister.py

@@ -146,7 +146,7 @@ class NugetLister(Lister[NugetListerState, NugetListerPage]):
                 )
                 continue
             xml = BeautifulSoup(res_metadata.content, "xml")
-            repo = xml.find("repository")
+            repo = xml.select_one("repository")
             if repo and "url" in repo.attrs and "type" in repo.attrs:
                 vcs_url = repo.attrs["url"]
                 vcs_type = repo.attrs["type"]