This simply updates the package dependencies by running flutter update-packages --force-upgrade.
I'm doing this with no other changes, because the last time I tried that, redness occurred. I want to isolate the problem to a "clean" update of the packages.
It looks like the plugins device_info, connectivity, and url_launcher haven't yet had their gradle configurations updated, so they fail when trying to build with the new gradle. I did not upgrade for those three packages only (in flutter_gallery) until we are ready to fix them (fixing them for master will break them for alpha users, so we need to do an alpha roll to do that).
This adds our self-compiled copy of the MinGit executable (built from the flutter/git repo) to the archive when building an archive for Windows.
I also tweaked the internal API for prepare_package.dart so that there's a single entry point to build an archive.
This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.
