From the doc:
command – Executes a command on a remote node
The command module takes the command name followed by a list of
space-delimited arguments. The given command will be executed on all
selected nodes. It will not be processed through the shell, so
variables like $HOME and operations like “<“, “>”, “|”, and “&” will
not work (use the shell module if you need these features).shell – Executes a commands in nodes
The shell module takes the command name followed by a list of space-delimited arguments.
It is almost exactly like the command module but runs the command
through a shell (/bin/sh) on the remote node.
Therefore you have to use shell: dpkg -l | grep python-apt.