Code review comment for ppa-dev-tools:fix-lp-test

"While working on running the test suite, I realized xdg is only used to fetch the home configuration directory. I wonder if there is a better way to find the lp credentials without depending on xdg. How does LP writes that file?"

It's a good question. The lp.py codebase is lifted from Bileto, which was placing the credentials right in ~/.launchpad.credentials. I added xdg to at least move this into ~/.config/ or whatever the user has specified as their config directory. However, if we just omit the credentials_file= argument then it'll pick an appropriate default. Robie mentioned this has the nice side effect of letting it share credentials with other LP-using tools.

Since that's a separate fix from these, I'll include that change in a followup branch.