[lit] Deduplicate README and longdescription, and update it

Since long_description is effectively the README that's displayed
on pypi project page, combine it with the existing README file and read
the file in `setup.py`.  This is a common practice among Python
projects, to the point of declarative-style setuptools configurations
providing a shorthand for it.

While at it, update the outdated information about LLVM Bugzilla for use
of GitHub issues.

Differential Revision: https://reviews.llvm.org/D137006