[CP] Fix analysis options discovery in the presence of workspaces

[pq]

See: #56552

This affects folks using multiple analysis options files w/ workspaces.

Risk of merge is low.

Change: https://dart-review.googlesource.com/c/sdk/+/403203

[dart-github-bot]

Summary: Workspace analysis options discovery is broken when multiple analysis options files are used. This change fixes the issue.

[athomas]

LGTM

[pq]

Thanks! And thanks for fixing up the whitespace in 46a20c1 @itsjustkevin!

[athomas]

Released in 3.6.1.