Rename and restructure the lib directory about approvaltests.cpp HOT 2 CLOSED

This will also extend nicely when catch3 is released.

from approvaltests.cpp.

This image shows what the cmake target dependencies look like now - there is a separate target for each 3rd-party dependency.

I created it using the commands given here - except saving in .png format instead of .svg:
https://github.com/claremacrae/project_dependencies_viz#alternative-approaches---if-you-use-cmake

The image in that post shows what the dependencies were in this project before today's changes, and before the addition of the addition of ApprovalTests_DocTest_Tests

from approvaltests.cpp.