internal: Get rid of the whole "sub cache" business. * Compute cache keys up front, so the cache directory is only chosen once. * Buffer up config variants to apply before loading the config. Makes it possible to cache variant-resulting configs, too. * Make a factory class to reuse the logic that creates the `PieCrust` object correctly for multi-process workers and such. * Add a test.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1