[pdf] Use implementation_deps for external deps

The pdf generation code depends on zlib and harfbuzz for implementation
reasons and does not re-export them or their headers. Using
implementation_deps means that the `includes` that these targets export
will not be exported to all dependents of the pdf targets.

Change-Id: Ie862a868b2e7cab3bb54e55d0f6d325fc55b60ff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1140917
Auto-Submit: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>