I already merged this change in #14057, but I'm doing it again because
our syncing process deleted the new files. It should be fixed now so
that it won't do that again.
Now that protobuf and upb share a Bazel repo, we can merge their
third_party/ directories. Because our syncing process doesn't know about
these files, there are multiple steps involved. This first step
duplicates lunit, the next step will update the code to use this new
copy, and then finally we can delete upb/third_party/lunit.
Our automation clobbered a few more files, so let's move them
temporarily so that we can put them back in place in a way that "git
blame" continues to work.
This is the second attempt to fix our Git history. This should allow
"git blame" to work correctly in the upb/ directory even though our
automation unexpectedly blew away that directory.