_emerge/depgraph.py: Autounmask-write fails when there isn't a file in package.*/ bug 598116

Instead of outputting "!!! No file to write for ..." error message,
Use a sane default filename.
Add zz- prefix to ensure it remains the last file in sorted order.

X-Gentoo-bug: 598116
X-Gentoo-bug-url: https://bugs.gentoo.org/598116
1 file changed