Limit flattened symbol links to key boundaries

[OskarEichler]

Summary

Only apply flattened symbol links to an exact key or a dotted descendant.

The prefix-only match currently allows a link for foo to rewrite an unrelated sibling such as foobar.

Verification

• focused external exact/nested/sibling link model
• 1,607 base-suite runs / 2,674 assertions
• 1,888 Rails-suite runs / 3,020 assertions
• all 46 shipped Ruby files pass syntax checks

[radar]

I don't understand the "why" of this change. Please add further description to the PR.