add subversion dependency support

This PR is much smaller than my previous attempt. The 'present' specification was removed and the 'svn' one simplified significantly.
Because svn URLs can specify trunk/branch/tag and revision, there is no separate specification of revision. This is documented briefly in README.md. I don't have any public svn material to make available in the packages file.
From distant memory, I'll wish you Joyeux Noël, et bonne Nouvelle Année.