implement /packages/{name} as its own route

A few things added with this commit:

- aurweb.packages.util
    - A module providing package and pkgbase helpers.
- aurweb.template.register_filter
    - A decorator that can be used to register a filter:
      @register_filter("some_filter") def f(): pass

Additionally, template partials have been split off a bit
differently. Changes:

- /packages/{name} is defined in packages/show.html.
- partials/packages/package_actions.html is now
  partials/packages/actions.html.
- partials/packages/details.html has been added.
- partials/packages/comments.html has been added.
- partials/packages/comment.html has been added.
- models.dependency_type additions: name and id constants.
- models.relation_type additions: name and id constants.
- models.official_provider additions: base official url constant.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/testing/html.py

@@ -0,0 +1,14 @@
+from io import StringIO
+
+from lxml import etree
+
+parser = etree.HTMLParser()
+
+
+def parse_root(html: str) -> etree.Element:
+    """ Parse an lxml.etree.ElementTree root from html content.
+
+    :param html: HTML markup
+    :return: etree.Element
+    """
+    return etree.parse(StringIO(html), parser)