Move Abseil from src to flutter third_party dir

[cbracken]

In combination with:

• https://flutter-review.googlesource.com/c/third_party/abseil-cpp/+/55848
• Move Abseil to flutter/third_party/abseil-cpp buildroot#831

this updates Flutter's references to Abseil from
//third_party/abseil-cpp to //flutter/third_party/abseil-cpp.

Issue: flutter/flutter#144201
Part of: flutter/flutter#67373

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• If eyes are exposed to patch: Hold eye open and rinse slowly and gently with water for 15-20 minutes. Remove contact lenses, if present, after the first 5 minutes, then continue rinsing eye.
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[cbracken]

This will fail presubmit due to the licence script; once that fails I'll check the results, patch and re-push.

[matanlurey]

test-exempt: just moving source paths

[auto-submit]

auto label is removed for flutter/engine/51245, due to - The status or check suite Linux linux_license has failed. Please fix the issues identified (or deflake) before re-applying this label.