Code review comment for lp:~al-maisan/launchpad/url-props-505382

Hello there!

This branch is converting the SPPH.(source|binary)_file_urls properties in to
converted into methods since the operations behind these access the database
and are too heavyweight for properties.

Tests to run:

    bin/test -vv -t xx-source-package-publishing

No (pertinent) "make lint" issues.