Add "automatic workspace discovery" to `transformChanges`

Created by: mrnugget

(Just replicating what I wrote in Slack here so we don't lose it)

While working on this [the "dynamic workspace discovery"] I’ve grown convinced that this “dynamic folder location discovery by using sourcegraph search” could/should also be used for transformChanges. In addition to directory: we could allow rootAtLocationOf and use the same mechanism to auto-discover the folders into which changes should be grouped.)

In more concrete terms: in addition to having this

transformChanges:
  group:
    - directory: client
      branch: hello-world-client # will replace the `branch` in the `changesetTemplate`
    - directory: docker-images
      repository: github.com/sourcegraph/sourcegraph
      branch: hello-world-infra

changesetTemplate:
  branch: hello-world # This branch is the default branch and will be overwritten for each additional changeset.
  # [...]

we could have something like this:

transformChanges:
  group:
    # Determine the directory locations using Sourcegraph search
    - rootAtLocationOf: package.json
      # Allow globbing when specifying the repository name
      in: github.com/sourcegraph/automation*
    - rootAtLocationOf: README.md
      # Stay backwards compatible:
      repository: github.com/sourcegraph/sourcegraph

changesetTemplate:
  # And we also use templating:
  branch: hello-world-${{ transformChanges.group.directory}}
  # Could also look like this:
  branch: hello-world-${{ changes.directory}}
  # [...]