ENH: put packages under once namespace and move to src/ repo layout

[pluflou]

For ease of use, consider putting all packages distributed under one same namespace (so they would instead become modules of a common package), for convention with most Python packages.

Also consider moving to src/ layout, benefits detailed here, e.g.,

• The src layout requires installation of the project to be able to run its code, and the flat layout does not.
• The src layout helps prevent accidental usage of the in-development copy of the code.