Agents have silent dependencies

I'm not 100% convinced this is the right way to go about it, as we've got multiple "things" using "AI" that have multiple ways of telling Drupal they need specific modules to do it and it feels like a lot of complexity and duplication: certainly with this change, if I understand what's going on (which I can't guarantee) the isAvailable functionality feels pointless - if the dependencies required aren't available, the plugin will not instantiate so isAvailable() can't be called.

But there's a MR for review and further discussion, anyway.

For 1.1.x/config agents, I think the isAvailable method is redundant and can be covered completely by the method that runs the agent. For the coded agents, this change is in theory good. but as Paul writes the method is now completely redundant for our current agents. It could however be that there is some contrib/private agent that uses this to check for an executable or some other none-module dependency conditional, so we should keep it in to not have breaking changes until 2.0.x at least.

Going forward its actually good if the the agent is allowed to run even if the necessary requirements are missing and then run into the tools exception or error message from execute and give back human readable reasons why it can't run a specific tool like "I can't seem to be able to get information about Views, because you haven't installed the Views module", "I can't look at your video, because FFmpeg is not installed on your server".

Tested and RTBC for me, so I'll go ahead an merge.