Simplify DESCRIPTION check

[malcolmbarrett]

This PR removes the tools-based DESCRIPTION file check and uses rprojroot to make a simpler check that the DESCRIPTION file exists in the project root and that it begins with ^Package. pkg_path() was already using rprojroot and was making this simpler check, so I beefed up that code a bit to make a clearer error message for the context.

This PR will simplify the use of loading non-package projects and closes #147.

Would it be helpful to add a test for a directory with an invalid pkg name and/or a directory without a DESCRIPTION file?

[jimhester]

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[jimhester]

And yes could you add a test case for this error, that would be great!

[malcolmbarrett]

Both taken care of, thanks! I added a test for the no DESCRIPTION file error + added a class to the abort() call for easier testing. I thought about trying to test loading with an invalid pkg name but decided it was a little obvious that wouldn't complain about it since it's no longer being checked.

[jimhester]