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.

---
in:
    pages/foo.md: |
        Foo!
    pages/bar.md: |
        Bar!
    pages/allpages.md: |
        {% for p in site.pages -%}
        {{p.url}}
        {% endfor %}
outfiles:
    allpages.html: |
        /
        /allpages.html
        /bar.html
        /foo.html