Generator lookup is managed by a list of
sources which return specs describing where to find and how to create
generators. This module provides class methods for manipulating the source
list and looking up generator specs, and an instance wrapper for quickly instantiating
generators by name.
A spec is not a generator: it‘s a description of where to find the
generator and how to create it. A source is anything that yields generators
from each. PathSource and GemGeneratorSource are provided.