Migrate namespace to support name change to rs-booster

[JBGreisman]

This PR begins the process of changing this package name to rs-booster. The idea is that this should be viewed as a "booster pack" to extend the functionality of reciprocalspaceship with helpful commandline tools/apps. As part of this move:

1. the top-level package name will be changed from efxtools->rs-booster
2. The Python package structure (top-level namespace) will be named from efxtools->rsbooster. To use rs-booster as a library in Python, one will use import rsbooster. This is sort of analogous to scikit-image/skimage.
3. All commandline tools will be installed to the rs.* namespace, rather than efxtools.*