The APF v13 `ng_package` rule will generate the `exports` field if not
set. Currently it allows for additional subpath entries to be configured
manually. The packager does not allow for custom conditions in subpath
exports which are auto-generated.
This is sometimes useful and necessary though. e.g. in Angular Material,
we also need to expose the index Sass file through a `sass` conditional
that the Webpack Sass loader will pick up. For this, the packager needs
to support manual additional conditions (as long as they do not
conflict).
PR Close#43764
