The actual source and header files come from the verbatim upstream checkout found at //third_party/zstd. This directory just provides the BUILD.gn file to stitch it into the Zircon build, and include/zstd/ with wrapper headers to redirect <zstd/zstd.h> and <zstd/zstd_seekable.h> to the real source location.

TODO(fxbug.dev/3156): After build unification, this might move to .../secondary/third_party/zstd so the //third_party/zstd path can be used in GN deps while still tracking the upstream repo unmodified.