Static package configuration

[markrtuttle]

This pull request makes two significant changes to how cbmc-viewer is packaged:

• It switches from dynamic to static configuration of setuptools (the second commit)
• It switches package building and installation from a manual invocation of setuptools to using the build and pip install packages (the third commit)

These are the preferred implementations described in the packaging overviews:

• Installing packages
• Packaging projects

It also moves the cbmc-viewer module under a src directory to be consistent with these documents (the first commit).

This pull requested is organized into three commits to make it easy to review, but they should be squashed into a single commit before merging.