Add missing package-system-test file, and update .gitignore
Disable a logical pathname equal test on clisp.
Gracefully handle the case where a package-system is missing a package.

This bug was found while testing with ECL, which somehow
evaluated the .asd twice during test-bundle.script
when other implementations evaluated it only once;
which revealed that the second time around,
the package-system would override the explicit secondary definitions
of test-asdf/dll-test and test-asdf/dll-user, and then
would fail ungracefully due to the absence of defpackage in these files.
That latent bug was present in all implementations,
but not stressed by the test system.
Bump version to 3.1.0.35