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Previously the Builder did not schedule file loads for configs of configs.

This meant that if a BUILD file contained only configs referenced by other
configs (rather than configs referenced by targets, or targets referenced by
other targets), it would never be loaded. The result will be a failed run with
an undefined object.

BUG=536844

Review URL: https://codereview.chromium.org/1373903002

Cr-Commit-Position: refs/heads/master@{#351089}