It's an obscure detail, but I think that the
.pyc file should not be rewritten again after we
fix the co_filenames.  Fixing the co_filenames
is a very very cheap operation, and I can imagine
cases where the same .py files are accessed from
what appears to be two different paths, e.g. over
NFS - this would cause .pyc files to be rewritten
all the time, which is particularly bad if we
have the example of NFS in mind.  Not to mention
that two python processes trying to write
*different* data to the same .pyc file at the
same time are going to create a mess, ending in
a segfault the next time the broken .pyc is
loaded.

It's overall a mess, so let's play it safe.