Add buftarget package

[doriable]

This PR adds a buftarget package, which provides path targeting information
for a storage bucket:

• BucketTargeting provides information on a ControllingWorkspace if one was
  found in the bucket, the input path, target paths, and target exclude paths
• TerminateAtControllingWorkspace has been consolidated to this package

The way the package is used is that when bufctl.Controller calls buffetch/internal.Reader
to get the storage bucket , buffetch/internal.Reader now also returns buftarget.BucketTargeting
alongside the bucket. bufworkspace.WorkspaceProvider then handles the workspace and module
targeting using buftarget.BucketTargeting. This removes the need to set input path, target paths,
and target exclude paths as workspace provider options, and is instead just passed through
workspaceTargeting and moduleTargeting.

This change also introduces some fallback logic in workspaceTargeting for inputs that did not
resolve to a controlling workspace.

Lastly, tests that were skipped previously for path/exclude-path behavior issues have been
unskipped and resolved accordingly.

[bufdev]

Approving to get this into bufmod as this is clearly an improvement over the status quo - we're going to do a line-by-line review as we go, but I've QA'ed the logic and done a basic pass, and we're in a much better state after this.