Bazel extractor: Promote input classification to a method.
Bazel extractor: Promote input classification to a method.

Summary: No functional change; this just makes it easier to reuse.

Reviewers: #core_team, salguarnieri

Reviewed By: #core_team, salguarnieri

Subscribers: salguarnieri, #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1786

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