taildrop: merge taildrop and feature/taildrop packages together

Fixes #15812

Change-Id: I3bf0666bf9e7a9caea5f0f99fdb0eb2812157608
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f