Add `importBundleForUrl` to merge bundles (useful as a stopgap for collections use)

[zachleat]

As described in 11ty/buildawesome#3730 any use of the HTML content from Collections items will not automatically inform the dependency graph (and include any dependent bundle contents).

It might be helpful to add a importBundleFromUrl or mergeBundleFromUrl method to merge bundles. This might be a bigger lift given that content does not include HTML from Eleventy Layouts (but the bundles as currently configured would?)

It might be easier to use eleventyImport.collections as a way to merge bundles too. https://www.11ty.dev/docs/data-configuration/#advanced