Support generating entire pipeline in a single step

[davidarcher]

When using this plugin in a monorepo, we end up with a lot of extra BuildKite jobs running just to do dynamic pipeline generation. The plugin kicks off a new Buildkite job/step for each matching entry in the watch list. It seems like it should be possible to generate the entire pipeline in a single step since we know the list of files that changed as soon as the build starts.

[davidarcher]

Maybe relevant alternative plugin for this use case: https://github.com/Zegocover/git-diff-conditional-buildkite-plugin