pkg_resources is on its way out, with importlib.resources and

importlib.metadata taking over much of the functionality.
This change updates our codebase to use these modern modules (by way of

backport modules depended on by Djblets) instead of pkg_resources.

The updates are largely straightforward, with the biggest change being a

migration for reviewboard.wsgi to remove the import and some old

multi-versioned install setup code originally contributed by Fedora.

Unit tests pass.
Verified that hosting services and SCMTools loaded by entrypoint were

available.
Tested upgrading older reviewboard.wsgi files.
Tested new sites with the updated reviewboard.wsgi file.
Tested the help output of rb-site to verify listed file paths.
Tested rbext create.