What's wrong with https://packages.debian.org/ and https://packages.ubuntu.com/ ?
The usability of searching "package directories" vs "the contents of packages", having to toggle between package names vs "source package names" vs descriptions, and choosing substring matching modes is... well, very typically Linux.
I prefer the search on the original sites, but this looks neat if you want the payload of a package.
Compare:
https://sources.debian.org/src/htop/2.0.2-1/
https://www.apt-browse.org/browse/debian/stretch/main/amd64/...
You can also search contents of packages if you select the option in search (the other options are package name, description)
For example https://tracker.debian.org/pkg/curl