caching: Use separate caches for config variants and other contexts. * The `_cache` directory is now organized in multiple "sub-caches" for different contexts. * A new context is created when config variants or overrides are applied. * `serve` context uses a different context that the other commends, to prevent the `bake` command's output from messing up the preview server (e.g. with how asset URLs are generated differently between the two). * Fix a few places where the cache directory was referenced directly.

syntax: glob
*.pyc
venv
tags
build/lib
build/messages/_cache
build/messages/_counter
dist
docs/_cache
docs/_counter
docs/bower_components
docs/node_modules
piecrust.egg-info
piecrust/__version__.py