Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
- Always append suffix `.yazi` to plugin's and flavor's attribute names.
- Warn if the attribute names already have the suffix.
- Assert that plugin's and flavor's values point to directories
containing an `init.lua` file.
- Append suffix `.yazi` to symlink targets of yazi plugins, if needed
- Improve some docs, especially links to upstream docs.
- Update tests to make use of this feature.
