Allow plugin name alias

[elliott-davis]

I hit an issue similar to https://github.com/buildkite/agent/issues/520 and solved it with the comment about using ssh://. While this totally works it gets pretty verbose as I use my plugin several times throughout the pipeline yaml.

Does anyone have thoughts on supporting a plugin alias with supporting config or env vars? Something like:

steps:
  - command: "echo whee"
     plugins:
       alias://awesome_plugin:
         ...

With a plugin alias list in the agent config file.

I feel like this is sort of a nit-pick issue but I'm happy to drop in a PR for it. I mostly wanted to take a quick pulse check on the style/appetite for a change like this before I wrote any code.

[AliSoftware]

This could also be interesting to handle plugin renames.

For example, we have created a Buildkite plugin internally to share a lot of commands that we use in many of our projects and pipelines. As the plugin grew in functionality, the original name we picked for the plugin does not reflect its purpose anymore, so we'd like to rename it… but many of our pipelines already reference it with the current name, so that may prove tricky to handle the transition (including if you take into consideration that our many projects also have multiple active branches)

With the ability to declare plugin aliases in the agents, we could consider creating an alias for the current name, then deploying it and update our pipelines to reference the plugin via its alias… and then when we're ready, rename the plugin repo and update the alias in the agents in one swoop, which would apply the change to all pipelines and client repos and branches at once.