Migrate `flutter/engine` to a Dart workspace for Dart/pub packages

[matanlurey]

tl;dr: Dart tooling is landing support for so-called Pub workspaces, the engine should use them when ready.

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Right now, flutter/engine has many, many pubspec.yaml file each of which needs to be independently maintained, and comply with our policy of not resolving any packages to external data sources (i.e., all packages have to either be vended by the Dart SDK, or via our own DEPS file).

The impact of this is perhaps most profound when you look at the necessary dependencies needed to use package:test as a dependency, which at the time of this writing, was nearly ~50 packages (and hand-written paths that resolve to different directories depending on the package).

With upcoming pub workspaces, we could instead:

• Have a single root flutter/engine/pubspec.yaml listing all pub dependencies we use.
• Have nested packages use resolution: workspace

Unblocks #133569.

See also:

• Implement workspaces [pub client] dart-lang/pub#4127
• Improve server performance by decreasing the number of Analysis context's created dart-lang/sdk#53874
• ☂️ Implement Workspace Feature dart-lang/sdk#53875