eng/common is frequent source of merge conflicts

1. I clone an Arcade repo at version x.
1. I need a fix from version y, so I do `darc update-dependencies`.
1. the master branch automatically updates to version z (skipping y).
1. I try to merge master back into my branch.
1. Merge conflicts happen all over eng/common.

We dealt with similar problems in KoreBuild and our solution was to minimize the amount of "boilerplate" checked into the repo by downloading it as a zip to a known location. Then the only boilerplate needed was the code to download things, which doesn't change very often.

CC @natemcmaster who I've discussed this with before.