Presubmit change detection is wrong for non-Bazel packages

[mattsoulanille]

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System information
The continuous integration docker image running presubmits.

Describe the current behavior
Presubmits for packages that do not build with Bazel are skipped when only Bazel dependencies change. This is because Bazel packages are not considered dependencies by package_dependencies.json.

Describe the expected behavior
Presubmits run on a package when its dependencies (including transitive dependencies) change.

Standalone code to reproduce the issue
#6329 did not test tfjs-backend-wasm (or any other non-Bazel packages) even though it changed tfjs-core.

Other info / logs Include any logs or source code that would be helpful to
diagnose the problem. If including tracebacks, please include the full
traceback. Large logs and files should be attached.

Cloudbuild logs for 6329

[google-ml-butler]

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