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JS modules for:
- //ui/file_manager/file_manager/background/js/
  - duplicate_finder.js
  - duplicate_finder_unitetst.js
- //u/file_manager/externs/background/duplicate_finder.js

Because Closure doesn't deal well importing the same namespace from
different files, I started splitting the "importer" namespace so each
file will have its own namespace.  Files in the "externs" directory will
have the suffix "Interface" on their namespace.  I'll use this logic for
all files related to "importer" namespace.

Bug: 1133186
Change-Id: I10240cb6d3083daf4bbc53773c391feb9d66c693
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596473
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837997}