PCScan: Move PCScanInternal into a separate file

This is only a refactoring change, no changes in logic.

Bug: 11297512
Change-Id: Id48a8b7aa5b412910e4d8ec9c2efd086a4b0a49c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2880664


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880939}