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This CL adds the core functionality of the polymer_modulizer() GN rule.
In follow up CLs, the js_modulizer() will be added, and some code will
be shared between the two tools.

Bug: 965770
Change-Id: I5f9e925c38d41d294e31877bfa7852e3fc5a88b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663734
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672330}