Add Related Commits for Owners

Summary:
For each commit, find the affected packages, and provide a way to
search by package.

Test Plan:
create commits that touch and don't touch two packages, and verify
that they display correctly in all the UI pages.

Reviewers: epriestley, blair, nh, tuomaspelkonen

Reviewed By: epriestley

CC: benmathews, aran, epriestley, btrahan, jungejason, mpodobnik, prithvi

Maniphest Tasks: T83

Differential Revision: 1208