title: APT says 'package has no installation candidate' althought it shows it in apt list
body: <p>I am trying to install <strong>ZeroMQ extension for PHP</strong> using Docker image <code>php:7.4-apache-buster</code>.</p>
  <p>The package is found when running <code>apt list</code>:</p>
  <p><code>$ sudo apt update &amp;&amp; apt list php-zmq</code> outputs <code>php-zmq/stable 1.1.3-9 amd64</code></p>
  <p>It is not possible to install it however:</p>
  <p><code>$ sudo apt install php-zmq</code> outputs <code>E: Package 'php-zmq' has no installation candidate</code></p>
  <p>I have checked that the package is not already installed using <code>apt list --installed php-zmq</code>. Underlying <code>libzmq5</code> library is present on the system.</p>
  <p>I wonder how does <code>apt list</code> command work that it suggests packages which <code>apt install</code> is unable to install.</p>
  <p>Thanks for any help.</p>