Move libpng to //flutter/third_party/libpng

[zanderso]

This is an example of moving a dependency to //flutter/third_party that requires forward/aliasing through the secondary build directory. Without forwarding, and only making changes to the DEPS file and rewriting paths in the engine repo, a gn invocation gets the following error:

ERROR at //third_party/freetype2/BUILD.gn:85:5: Can't load input file.
    "//third_party/libpng",
    ^---------------------
Unable to load:
  /Users/zra/flutter/engine/src/third_party/libpng/BUILD.gn
I also checked in the secondary tree for:
  /Users/zra/flutter/engine/src/flutter/build/secondary/third_party/libpng/BUILD.gn

Another dependency, with a BUILD.gn file outside of the engine repo, is still referring to libpng by the old path. However, gn tells us that it is also checking under the secondary tree. We can put whatever we want in that BUILD.gn file to make freetype2 happy. In this case it's easy, we just add a build rule there that forwards to libpng's new location. There's only one wrinkle, which is that the dependency in the group target has to be public_deps rather than a simple deps. This is so that public_deps and public_configs of //flutter/third_party/libpng are correctly inherited by dependents of //third_party/libpng.

[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.

[matanlurey]

TIL. Thanks Zach, I'll try applying this once and see if I get it

(and I'll add it to the FAQ).

[flutter-dashboard]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

If you are still iterating on this change and are not ready to resolve the images on the Flutter Gold dashboard, consider marking this PR as a draft pull request above. You will still be able to view image results on the dashboard, commenting will be silenced, and the check will not try to resolve itself until marked ready for review.

Changes reported for pull request #50571 at sha 01b185b