Make use of md5_check within package_resources.py
Make use of md5_check within package_resources.py

Turns the step into a no-op when none of its real dependencies have changed
(e.g. don't re-run it when a .java file changes)

BUG=533442

Review URL: https://codereview.chromium.org/1351033003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#350764}

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