Add test that detects relative imports in packages

[jelbourn]

Related to #12876

TypeScript 2.9 has the capability to generate inline imports that are using relative imports. This is bad because our package structure does not match our source structure, meaning that those relative imports would break downstream apps.

Ideally we would have a type of e2e test that validates our packages actually work, but until then we can at least sanity check this in a new CI task.

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