Adopt namespace naming guidelines - part 1

[mlw]

The style guide prefers that a projects do not overly nest namespace names, and note that unless necessary, a single top-level namespace per project is the most effective strategy to limit exposure to collisions.

To keep PR size reasonable, part 1 only changes things in santa::common and santa::santad::data_layer. Other namespaces will be converted in a future PR.

References:

• Namespaces style guide
• Namespace names style guide
• Tip of the Week #130: Namespace Naming