design: add proposal doc for 33974
design: add proposal doc for 33974

Make the `cmd/internal/lockedfile` Package public.

Updates golang/go#33974

Change-Id: I2502fad153254d9ddb2bcc96ed6d8ef163940add
GitHub-Last-Rev: d6be79e2bc2e232ab1ed7990ec36219f8b427040
GitHub-Pull-Request: golang/proposal#21
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/201277
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/proposal,golang/proposal