The direct answer is
procps. Here is how you can find this out for yourself:
# Install apt-file, which allows you to search
# for the package containing a file
sudo apt-get install apt-file
# Update the package/file mapping database
sudo apt-file update
# Search for "top" at the end of a path
apt-file search --regexp '/top$'
The output of the final command should look something like this:
You can see that only
procps provides an executable in your standard PATH, which gives a clue that it might be the right one. You can also find out more about
procps to make sure like it seems like the right one:
$ apt-cache show procps
Description-en: /proc file system utilities
This package provides command line and full screen utilities for browsing
procfs, a "pseudo" file system dynamically generated by the kernel to
provide information about the status of entries in its process table
(such as whether the process is running, stopped, or a "zombie").
It contains free, kill, pkill, pgrep, pmap, ps, pwdx, skill, slabtop,
snice, sysctl, tload, top, uptime, vmstat, w, and watch.