Populate source location in summary page

[mikepurvis]

Currently it's just ?:

https://github.com/mikepurvis/catkin_tools_document/blob/3ed28b171eabb7ebfa31856fe025177fc6c03ac0/catkin_tools_document/messages.py#L138

Two possibilities for where to get this information:

• Use the default remote from the directories in the workspace (only works if they're git repos and not, eg, tarballs).
• Using ROSDISTRO_INDEX_URI, query the rosdistro source cache (only works if the source information is part of the cache).

[MatthijsBurgh]

This is semi fixed by https://github.com/mikepurvis/catkin_tools_document/pull/14